Make oom_score_adjust_is_valid() static

It has only one user and we don't need to put it in basic/.

src/basic/process-util.c

@@ -1061,10 +1061,6 @@ _noreturn_ void freeze(void) {
                 pause();
 }
 
-bool oom_score_adjust_is_valid(int oa) {
-        return oa >= OOM_SCORE_ADJ_MIN && oa <= OOM_SCORE_ADJ_MAX;
-}
-
 unsigned long personality_from_string(const char *p) {
         int architecture;