After upgrade from 16.04 LTS (ESM with disabled UA) to 18.04 LTS (supported) all mentions of ESM should be removed from the system

It is really strange to have ESM enabled on the 18.04 LTS which is supported.

Currently I have:

```
$ dpkg -S esm | grep ubuntu-advantage-tools
ubuntu-advantage-tools: /usr/lib/ubuntu-advantage/apt-esm-hook
ubuntu-advantage-tools: /usr/share/keyrings/ubuntu-advantage-esm-apps.gpg
ubuntu-advantage-tools: /etc/update-motd.d/91-contract-ua-esm-status
ubuntu-advantage-tools: /usr/lib/ubuntu-advantage/apt-esm-json-hook
ubuntu-advantage-tools: /usr/lib/python3/dist-packages/uaclient/entitlements/esm.py
ubuntu-advantage-tools: /etc/apt/apt.conf.d/20apt-esm-hook.conf
ubuntu-advantage-tools: /usr/share/keyrings/ubuntu-advantage-esm-infra-trusty.gpg
ubuntu-advantage-tools: /etc/update-motd.d/88-esm-announce

$ sudo grep -r esm /etc/
/etc/update-motd.d/88-esm-announce:stamp="/var/lib/ubuntu-advantage/messages/motd-esm-announce"
/etc/update-motd.d/91-contract-ua-esm-status:stamp="/var/lib/ubuntu-advantage/messages/motd-esm-service-status"
/etc/ubuntu-advantage/help_data.yaml:esm-apps:
/etc/ubuntu-advantage/help_data.yaml: its end of life. You can find out more about the esm service at
/etc/ubuntu-advantage/help_data.yaml: https://ubuntu.com/security/esm
/etc/ubuntu-advantage/help_data.yaml:esm-infra:
/etc/ubuntu-advantage/help_data.yaml: esm-infra provides access to a private ppa which includes available high
/etc/ubuntu-advantage/help_data.yaml: You can find our more about the esm service at
/etc/ubuntu-advantage/help_data.yaml: https://ubuntu.com/security/esm
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:deb https://esm.ubuntu.com/infra/ubuntu xenial-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:deb https://esm.ubuntu.com/infra/ubuntu xenial-infra-updates main
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-updates main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:deb https://esm.ubuntu.com/infra/ubuntu bionic-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:deb https://esm.ubuntu.com/infra/ubuntu bionic-infra-updates main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-updates main
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-hook ] || /usr/lib/ubuntu-advantage/apt-esm-hook post-invoke-stats || true";
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-hook ] || /usr/lib/ubuntu-advantage/apt-esm-hook post-invoke-success || true";
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-hook ] || /usr/lib/ubuntu-advantage/apt-esm-hook pre-invoke || true";
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-json-hook ] || /usr/lib/ubuntu-advantage/apt-esm-json-hook || true";
Binary file /etc/apt/trusted.gpg.d/ubuntu-advantage-esm-infra-trusty.gpg matches

```

Having `https://esm.ubuntu.com/infra/ubuntu bionic-infra-updates` in sources.list is strange.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-advantage-tools 27.0.2~18.04.1
ProcVersionSignature: Ubuntu 4.15.0-143.147-generic 4.15.18
Uname: Linux 4.15.0-143-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CurrentDesktop: MATE
Date: Sat May 15 10:59:54 2021
InstallationDate: Installed on 2018-08-07 (1011 days ago)
InstallationMedia: Ubuntu-MATE 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
SourcePackage: ubuntu-advantage-tools
UpgradeStatus: Upgraded to bionic on 2021-05-15 (0 days ago)