Remove __isnanl() to avoid conflicting with the same function in libm.

__isnan() and __isnanf() must remain in libc for hysterical raisins.
On the other hand, __isnanl() must live in libm because libm uses it
internally and can't depend on older versions of libc to provide it.
Fortunately, we don't need __isnanl() in both libraries.

Prodded by:	ale
PR:		71698
MT5 candidate

lib/libc/gen/isnan.c

@@ -55,13 +55,3 @@ __isnanf(float f)
 	u.f = f;
 	return (u.bits.exp == 255 && u.bits.man != 0);
 }
-
-int
-__isnanl(long double e)
-{
-	union IEEEl2bits u;
-
-	u.e = e;
-	mask_nbit_l(u);
-	return (u.bits.exp == 32767 && (u.bits.manl != 0 || u.bits.manh != 0));
-}