Drop package 'p7zip'

The Git for Windows project uses `mingw-w64-7zip` these days, because p7zip is stuck way in the past, see also git-for-windows/git#5395.

Here is the output of the command that was run in
https://github.com/git-for-windows/git-for-windows-automation/actions/runs/27194161241:

+ set -x
+ pacman -Rns --noconfirm p7zip
checking dependencies...

Packages (1) p7zip-17.06-1

Total Removed Size:  10.82 MiB

:: Do you want to remove these packages? [Y/n]
:: Processing package changes...
removing p7zip...
+ echo 0

Author: Git for Windows Build Agent