Remove module for xlocale.h

As commit 2db042523a explains, this header doesn't exist anymore since
glibc 2.26. The oldest distributions that ROOT currently supports are
EL8 and Debian 10, both of which have glibc 2.28. Remove the module as
it also causes problems on FreeBSD due to including locale.h and a
cyclic dependency std -> xlocale.h -> std.

include/cling/libc.modulemap

@@ -62,13 +62,4 @@ module "libc" [system] [extern_c] [no_undeclared_includes] {
     export *
     header "wchar.h"
   }
-
-  use "xlocale.h"
-}
-
-// glib specific header. In it's own module because it
-// doesn't exist on some systems with unpatched glib 2.26+
-module "xlocale.h" [system] [extern_c] {
-  export *
-  header "xlocale.h"
 }