fs/binfmt_elf: remove redundant elf_map ifndef

The ifndef was added a long time ago to support archs that would define
their own mapping function.  The last user was the metag arch which was
removed from the tree, and as such there are no users left.  Let's kill
it.

Signed-off-by: Anthony Iliopoulos <ailiop@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200402161543.4119-1-ailiop@suse.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>