Fix redundant return statements

… at the end of a function with a void return type.

Found using clang-tidy's readability-redundant-control-flow.

i3bar/src/xcb.c

@@ -2067,5 +2067,4 @@ void set_current_mode(struct mode *current) {
     I3STRING_FREE(binding.name);
     binding = *current;
     activated_mode = binding.name != NULL;
-    return;
 }