ARC, thp: remove infrastructure for handling splitting PMDs

With THP refcounting work, no need to mark PMDs splitting.

(ARC got missed under the sweeping arch change as THP support was likely
not present in orig baseline)

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>