small cleanup in gpt partition handling

Remove unused argument in is_pmbr_valid()
Remove unneeded initialization of local variable legacy_mbr

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>