Remove alpha math_private.h.

After some math_private.h cleanups (in particulat math-barriers.h
being split out), the only thing left in the alpha math_private.h was
macro definitions of __isnan and __isnanf, apparently (based on the
comments) intended to avoid problems with inline definitions in other
math_private.h files.  Those inline definitions were removed in commit
fe8c2b33, and the alpha math_private.h
is no longer needed; this patch removes it.

Tested with build-many-glibcs.py that installed stripped shared
libraries for alpha are unchanged by the patch.

	* sysdeps/alpha/fpu/math_private.h: Remove.