misc: Remove support for checking out as a mercurial repo.

misc: Remove support for checking out as a mercurial repo.

This will still be technically possible with the right converters, but
this removes the tags, ignore file, and style checking hooks related to
mercurial. We no longer maintain a mercurial mirror of the main git
repository, and this support adds clutter and could diverge from the git
style hooks, etc, over time.

Change-Id: Icf4833c4f0fda51ea98989d1d741432ae3ddc6dd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31174
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>