pam_systemd: move socket_from_display() from util.[ch] to pam_systemd.c

It's highly specific, kinda legacy (X11…) and only used at one place,
let's move this out of the common code, and into pam_systemd.c where it
is used.

src/basic/util.c

@@ -77,31 +77,6 @@ bool display_is_local(const char *display) {
                 display[1] <= '9';
 }
 
-int socket_from_display(const char *display, char **path) {
-        size_t k;
-        char *f, *c;
-
-        assert(display);
-        assert(path);
-
-        if (!display_is_local(display))
-                return -EINVAL;
-
-        k = strspn(display+1, "0123456789");
-
-        f = new(char, STRLEN("/tmp/.X11-unix/X") + k + 1);
-        if (!f)
-                return -ENOMEM;
-
-        c = stpcpy(f, "/tmp/.X11-unix/X");
-        memcpy(c, display+1, k);
-        c[k] = 0;
-
-        *path = f;
-
-        return 0;
-}
-
 bool kexec_loaded(void) {
        _cleanup_free_ char *s = NULL;