UPSTREAM: arm64: remove dead code

Commit 68234df4ea79 ("arm64: kill flush_cache_all()") removed
soft_reset() from the kernel. This was the only caller of
setup_mm_for_reboot(), so remove that also.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

Bug: 31432001
Change-Id: Ic138d2e53449c73bf4d289f5e33bb04588e40baf
(cherry picked from commit b08d4640a3dca68670fc5af2fe9205b395a02388)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>