Remove nan() fallback

We stopped using nan() when @krader1961 changed the timef() function
a while back. I removed the autoconf check recently as well.

src/fallback.cpp

@@ -255,10 +255,6 @@ int killpg(int pgr, int sig) {
 }
 #endif
 
-#ifndef HAVE_NAN
-double nan(char *tagp) { return 0.0 / 0.0; }
-#endif
-
 // Big hack to use our versions of wcswidth where we know them to be broken, which is
 // EVERYWHERE (https://github.com/fish-shell/fish-shell/issues/2199)
 #ifndef HAVE_BROKEN_WCWIDTH