LibJS: Remove Cell::interpreter()

It's never necessary to find the current Interpreter for a given Cell
anymore. Get rid of this accessor.

Libraries/LibJS/Runtime/Cell.cpp

@@ -56,16 +56,6 @@ VM& Cell::vm() const
     return heap().vm();
 }
 
-Interpreter& Cell::interpreter()
-{
-    return heap().interpreter();
-}
-
-Interpreter& Cell::interpreter() const
-{
-    return heap().interpreter();
-}
-
 const LogStream& operator<<(const LogStream& stream, const Cell* cell)
 {
     if (!cell)

Libraries/LibJS/Runtime/Cell.h

@@ -61,8 +61,6 @@ public:
 
     Heap& heap() const;
     VM& vm() const;
-    Interpreter& interpreter();
-    Interpreter& interpreter() const;
 
 protected:
     Cell() { }