Remove apt-get install instructions from README.md

We are no longer deploying debian packages to Microsoft feeds. Further, all
previous deployments have been removed from the applicable feeds. Removing
the instructions for installation via apt-get in light of these changes.

Author: ldennington

README.md

@@ -88,33 +88,7 @@ sudo /usr/local/share/gcm-core/uninstall.sh
 <a name="linux-install-instructions"></a>
 ### Linux
 
-#### Debian package (.deb)
-
-`apt-get` support is available for Ubuntu Bionic Beaver (18.04) and Hirsute 
-Hippo (21.04). Take the following steps to set up and install based on the
-version you are running:
-
-##### Ubuntu 18.04 (Bionic)
-
-```shell
-curl -sSL https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -
-sudo apt-add-repository https://packages.microsoft.com/ubuntu/18.04/prod
-sudo apt-get update
-sudo apt-get install gcmcore
-git-credential-manager-core configure
-```
-
-##### Ubuntu 21.04 (Hirsute)
-
-```shell
-curl -sSL https://packages.microsoft.com/config/ubuntu/21.04/prod.list | sudo tee /etc/apt/sources.list.d/microsoft-prod.list
-curl -sSL https://packages.microsoft.com/keys/microsoft.asc | sudo tee /etc/apt/trusted.gpg.d/microsoft.asc
-sudo apt-get update
-sudo apt-get install gcmcore
-git-credential-manager-core configure
-```
-
-##### Other Ubuntu/Debian distributions
+#### Ubuntu/Debian distributions
 
 Download the latest [.deb package](https://github.com/microsoft/Git-Credential-Manager-Core/releases/latest), and run the following: