MIPS: Remove leftovers from the IRIX binary compat code.

2957c9e6

 (kernel.org) rsp.
b934da913f236bca00c41d9e386e980586000461 (lmo) [[MIPS] IRIX: Goodbye and
thanks for all the fish] left two fields in struct thread_struct which
were only being used for the IRIX compat code.  Remove them.

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