service: drop _pure_ decorator on static function

The compiler should be good enough to figure this out on its own if this
is a static function, and it makes control_pid_good() an outlier anyway,
and decorators like this tend to bitrot. Hence, to keep things simple
and automatic, let's just drop the decorator.

src/core/service.c

@@ -1422,7 +1422,7 @@ static int main_pid_good(Service *s) {
         return -EAGAIN;
 }
 
-_pure_ static int control_pid_good(Service *s) {
+static int control_pid_good(Service *s) {
         assert(s);
 
         /* Returns 0 if the control PID is dead, > 0 if it is good. We never actually return < 0 here, but in order to