Drop package 'git-flow'

The Git Flow package is obsolete and unmaintained. It is no longer distributed with Git for Windows, see git-for-windows/build-extra#648 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/18565589197:

+ set -x
+ pacman -Rns --noconfirm git-flow
checking dependencies...

Packages (1) git-flow-1.12.3-2

Total Removed Size:  0.22 MiB

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

Author: Git for Windows Build Agent