pip: remove the snap in addition to OS packages

[alexzorin]

Some minor edits to the instructions to:

• Ask the user to remove the Certbot snap in addition to OS packages and certbot-auto
• Remove some irrelevant copy ("... before installing the Certbot snap ...") which was copy-pasted from the snapd instructions.

This came up on the forums. I guess some users will move from snapd to pip on occasion. It especially makes sense if they want a third-party plugin which isn't snapped.

[alexzorin]

One day I will read a grammar guide :) .

[bmw]

Closing and reopening to trigger CI.

[ohemorange]

This code is no longer being tracked on this repo, but I'll send the suggestions over to the team that owns the codebase now.