Simplify INFINITY definitions.

Simplify INFINITY definitions.

Similar to my patch for HUGE_VAL, this patch eliminates the bits/inf.h
headers and just unconditionally uses the same definitions as the
sysdeps/ieee754 version did (__builtin_inff () for GCC >= 3.3,
otherwise HUGE_VALF), directly in math.h, so removing an unnecessary
level of indirection.

Tested for x86_64.

	* math/math.h [__USE_ISOC99] (INFINITY): Define directly here.  Do
	not include <bits/inf.h>.
	* math/Makefile (headers): Remove bits/inf.h.
	* bits/inf.h: Remove.
	* sysdeps/ieee754/bits/inf.h: Likewise.