remove significandl

this function never existed historically; since the float/double
functions it's based on are nonstandard and deprecated, there's really
no justification for its existence except that glibc has it. it can be
added back if there's ever really a need...

include/math.h

@@ -403,7 +403,6 @@ long double pow10l(long double);
 
 double      significand(double);
 float       significandf(float);
-long double significandl(long double);
 #endif
 
 #ifdef __cplusplus

src/math/significandl.c

-#define _GNU_SOURCE
-#include <math.h>
-
-long double significandl(long double x)
-{
-	return scalbnl(x, -ilogbl(x));
-}