MIPS: 32-bit: Remove unused gas macros fifty and mille.

These are a leftover of the IRIX compat code which was removed in
2957c9e6

 (kernel.org) rsp.
b934da913f236bca00c41d9e386e980586000461 (lmo) [[MIPS] IRIX: Goodbye
and thanks for all the fish].

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>