Remove Ubuntu PPA installation instructions
Remove Ubuntu PPA installation instructions

Since the Ubuntu PPA from mmstick is apparently not maintained anymore,
people should no longer be instructed to install the outdated version
that is available from the PPA.

The System76 PPA repository still contains an up to date version of
Alacritty, but since it also comes with several other packages we cannot
recommend users to install it outside of Pop!_OS.

Fixes #3863.
alacritty/alacritty,alacritty/alacritty,jwilm/alacritty,jwilm/alacritty,jwilm/alacritty,jwilm/alacritty