Remove pre-GCC-4.9 MIPS code.

This patch removes some MIPS code in glibc that was conditional on old
GCC versions no longer supported for building glibc.

Tested with build-many-glibcs.py.

	* sysdeps/mips/atomic-machine.h (R10K_BEQZ_INSN): Remove.
	[__GNUC_PREREQ (4, 8) || __mips16]: Make code unconditional.
	[!__GNUC_PREREQ (4, 8) && !__mips16]: Remove conditional code.
	* sysdeps/mips/math-tests.h
	[_MIPS_SIM != _ABIO32 && !__GNUC_PREREQ (4, 9)]: Remove
	conditional code.