Remove unused check in heap_xlog_insert()

Remove unused check in heap_xlog_insert()

8e03eb92e9a reverted the commit 39b66a91bd which allowed freezing
in the heap_insert() code path but forgot to remove the corresponding
check in heap_xlog_insert(). This code is extraneous but not harmful.
However, cleaning it up makes it very clear that, as of now, we do not
support any freezing of pages in the heap_insert() path.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Tomas Vondra <tomas@vondra.me>
Discussion: http://postgr.es/m/flat/CAAKRu_Zp4Pi-t51OFWm1YZ-cctDfBhHCMZ%3DEx6PKxv0o8y2GvA%40mail.gmail.com
Backpatch-through: 14