automake: don't mandate git_sha1.h/MESA_GIT_SHA1

automake: don't mandate git_sha1.h/MESA_GIT_SHA1

It has proven subtle to get it right both from the build side POV (see
commit list below) and builders due to their varying workflows.

Furthermore it does not fully fulfil the reason why it was enforced -
to detect uniqueness between different builds, in order to distinguish
and invalidate Vulkan/GL caches.

With that having a much better solution (previous commit) we can drop
this solution.

This effectively reverts the following commits:
359d9dfec33 ("mesa: automake: add directory prefix for git_sha1.h")
2c424e00c39 ("mesa: automake: ensure that git_sha1.h.tmp has the right
attributes")
b7f7ec78435 ("mesa: automake: distclean git_sha1.h when building OOT")
8229fe68b5d ("automake: get in-tree `make distclean' working again.")

Cc: Timo Aaltonen <tjaalton@debian.org>
Cc: Haixia Shi <hshi@chromium.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
(cherry picked from commit 22e9357028f45fab2563bdf2f3e171d4490a5d05)