AK: Remove must_set() from JsonArray

Due to 582c55a, both `must_set()` and `set()` should be providing the
same behavior. Not only is that a reason to remove `must_set()`, but
it also performs erroneous behavior since it inserts an element at
a specified index instead of modifying an element at that index.

AK/JsonArray.h

@@ -62,7 +62,6 @@ public:
     [[nodiscard]] JsonValue take(size_t index) { return m_values.take(index); }
 
     void must_append(JsonValue value) { m_values.append(move(value)); }
-    void must_set(size_t index, JsonValue value) { m_values.insert(index, move(value)); }
 
     void clear() { m_values.clear(); }
     ErrorOr<void> append(JsonValue value) { return m_values.try_append(move(value)); }