xtensa: drop extraneous register load from initialize_mmu

Commit a9f2fc62 ("xtensa: cleanup MMU setup and kernel layout macros")
removed the use of a2 in the beginning of the initialize_mmu macro, but
left the register load that is no longer used. Remove it as well.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>