include/linux/huge_mm.h: remove mincore_huge_pmd declaration

As mincore_huge_pmd() was dropped, remove the declaration from the header
file.

Signed-off-by: Yulei Zhang <yuleixzhang@tencent.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Link: https://lkml.kernel.org/r/20200922083423.15074-1-yuleixzhang@tencent.com


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