MIPS: Replace regset_alloc() invocations by static regset structures.

After removal of the regset_alloc invocations, the appropriate tdep
fields become obsolete and are thus removed.

gdb/mips-tdep.h

@@ -109,14 +109,6 @@ struct gdbarch_tdep
   int register_size_valid_p;
   int register_size;
 
-  /* General-purpose registers.  */
-  struct regset *gregset;
-  struct regset *gregset64;
-
-  /* Floating-point registers.  */
-  struct regset *fpregset;
-  struct regset *fpregset64;
-
   /* Return the expected next PC if FRAME is stopped at a syscall
      instruction.  */
   CORE_ADDR (*syscall_next_pc) (struct frame_info *frame);