compat_binfmt_elf: don't bother with undef of ELF_ARCH

It's not used anywhere downstream (and never had been, AFAICS).
Theoretically, fs/binfmt_elf.c does use it, but only in the
non-regset coredump handling and all biarch architectures
end up with that ifdefed out.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/compat_binfmt_elf.c

@@ -62,7 +62,6 @@
  * differ from the native ones, or omitted when they match.
  */
 
-#undef	ELF_ARCH
 #undef	elf_check_arch
 #define	elf_check_arch	compat_elf_check_arch