Kill ancient crap in s390 compat mmap

We've had TASK_SIZE set to 1<<31 for 31bit tasks since May 2004.
Before that old32_mmap() had to deal with do_mmap_pgoff() giving
it an address out of range.  It had tried to do that by checking
return value and doing do_munmap() (at wrong address, BTW).

IOW, that code had been dead for 5.5 years (and bogus - for 8).
Kill.

Acked-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>