SCI: Avoid incrementing lockers of deleted script

Having a deleted script with non-zero lockers had the side effect of
making the deleted script re-appear in the GC's work list, including
any (deleted) objects in the script.

This should be the root cause of bugs #3299458 and #3295849, so
also delete the workaround added for that in 35086fe1.

engines/sci/engine/script.cpp

@@ -380,6 +380,7 @@ void Script::relocateSci3(reg_t block) {
 }
 
 void Script::incrementLockers() {
+	assert(!_markedAsDeleted);
 	_lockers++;
 }
 
@@ -393,6 +394,7 @@ int Script::getLockers() const {
 }
 
 void Script::setLockers(int lockers) {
+	assert(lockers == 0 || !_markedAsDeleted);
 	_lockers = lockers;
 }