Remove duplicate inline implementation of issignalingf

Very recent commit 854e91bf enabled
inline of issignalingf() in general (__issignalingf in include/math.h).
There is another implementation for an inline use of issignalingf
(issignalingf_inline in sysdeps/ieee754/flt-32/math_config.h)
which could instead make use of the new enablement.

Replace the use of issignalingf_inline with __issignaling.  Using
issignaling (instead of __issignalingf) will allow future enhancements
to the type-generic implementation, issignaling, to be automatically
adopted.

The implementations are slightly different, and compile to slightly
different code, but I measured no significant performance difference.

The second implementation was brought to my attention by:
Suggested-by: Joseph Myers <joseph@codesourcery.com>
Reviewed-by: Joseph Myers <joseph@codesourcery.com>