Remove unused sparc64/soft-fp libm function implementations.
sysdeps/sparc/sparc64/soft-fp has files s_frexpl.c, s_scalblnl.c and
s_scalbnl.c that are never used because the ldbl-128 versions always
come first in the sysdeps directory ordering. This patch removes the
unused files.
Tested with build-many-glibcs.py that installed stripped shared
libraries are unchanged by this patch.