LibJS: Fix Map Iterators when elements are deleted during iteration

Before this would assume that the element found in operator++ was still
valid when dereferencing it in operator*.
Since any code can have been run since that increment this is not always
valid.
To further simplify the logic of the iterator we no longer store the
index in an optional.

Userland/Libraries/LibJS/Runtime/Map.cpp

@@ -34,7 +34,7 @@ bool Map::map_remove(Value const& key)
 {
     Optional<size_t> index;
 
-    for (auto it = m_keys.begin(); it != m_keys.end(); ++it) {
+    for (auto it = m_keys.begin(); !it.is_end(); ++it) {
         if (ValueTraits::equals(*it, key)) {
             index = it.key();
             break;