Revert "ac/nir: Lower large indirect variables to scratch"

This reverts commit 74470bae.

This change introduces some significant performance regressions. We
are fixing those on master, but the follow up work is large enough
not to backport to 19.2 .

Fixes: 74470bae "ac/nir: Lower large indirect variables to scratch"
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111576


Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>