xtensa: don't clear swapper_pg_dir in paging_init

swapper_pg_dir is located in the .bss, so it's zero-initialized anyway.
With KASAN enabled paging_init will be called after KASAN
initialization, it must not erase page directory entries set up for
KASAN shadow map.

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