docs: Remove Arch from the release process.

On December 6th 2024, the fex-emu packages got a deletion request:

> MarsSeed [1] filed a deletion request for fex-emu [2]:
>
> ARM-only package.
> This should be submitted to ArchLinuxARM.org [a], not to AUR - see
> quote from ArchWiki [b]:
>
>     "Packages that do not support the x86_64 architecture
>     are not allowed in the AUR."
>
> [a]:
> https://archlinuxarm.org/forum/viewforum.php?f=4
> [b]:
> https://wiki.archlinux.org/title/AUR_submission_guidelines#Rules_of_submission
>
> [1] https://aur.archlinux.org/account/MarsSeed/
> [2] https://aur.archlinux.org/pkgbase/fex-emu/

This is due to a rule clarification that occured in Arch's forum on November 25th: https://lists.archlinux.org/archives/list/aur-general@lists.archlinux.org/thread/IRZ2LWYX3ECPJQZJXMLAP6JIKL6HLHPZ/#GMYC74CRSFH7GGNENEUOODZUPWHOMX7A

On December 3rd the package submission guidelines on their wiki was
updated to mandate x86-64 support:
https://wiki.archlinux.org/index.php?title=AUR_submission_guidelines&diff=prev&oldid=822050

As of today, December 7th, 2024 the packages have been removed from AUR
due to only supporting aarch64.

> Muflone [1] deleted fex-emu [2].
>
> You will no longer receive notifications about this package.
>
> [1] https://aur.archlinux.org/account/Muflone/
> [2] https://aur.archlinux.org/pkgbase/fex-emu/

ArchLinux is no longer a supported distro for FEX, remove it from the release processes documentation.

docs/ReleaseProcess.md

@@ -40,12 +40,6 @@ Follow the steps in: https://github.com/FEX-Emu/FEX-ppa/blob/main/README.md
 * Requires PPA GPG key signing access
 * Wait the 20-30 minutes for Ubuntu PPA to build and publish the binaries
 
-## ArchLinux AUR package
-* Clone https://aur.archlinux.org/packages/fex-emu
-  * Requires maintainer or co-maintainer permissions
-* Update PKGBUILD and .SRCINFO file.
-* Push changes upstream
-
 ## Github releases page Steps
 * Requires administrative rights
 * Go to https://github.com/FEX-Emu/FEX/releases