hexagon: drop the unused variable zero_page_mask

Hexagon arch does not seem to have subscribed to _HAVE_COLOR_ZERO_PAGE
framework.  Hence zero_page_mask variable is not needed.

Link: http://lkml.kernel.org/r/20180517061105.30447-1-khandual@linux.vnet.ibm.com


Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>