Remove alpha nearbyint wrapper.

Given my recent changes, sysdeps/alpha/fpu/s_nearbyint.c is no longer
needed: it just includes the dbl-64/wordsize-64 version, which is the
one that would be used anyway, and defines a compat symbol,
duplicating the same compat symbol defined by the dbl-64/wordsize-64
version through use of libm_alias_double.  Thus, this patch removes
the redundant wrapper.

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

	* sysdeps/alpha/fpu/s_nearbyint.c: Remove file.