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.h

@@ -50,7 +50,6 @@ static inline const char* enable_disable(bool b) {
 bool plymouth_running(void);
 
 bool display_is_local(const char *display) _pure_;
-int socket_from_display(const char *display, char **path);
 
 #define NULSTR_FOREACH(i, l)                                    \
         for ((i) = (l); (i) && *(i); (i) = strchr((i), 0)+1)