Drop package 'unrar'

The Git for Windows project never used `unrar`. See git-for-windows/git#5395 for details.

Here is the output of the command that was run in
https://github.com/git-for-windows/git-for-windows-automation/actions/runs/27203180380:

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

Packages (1) unrar-7.2.6-1

Total Removed Size:  0.36 MiB

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

Author: Git for Windows Build Agent