LibJS/Bytecode: Leave BlockDeclarationInstantiation in C++

Instead of implementing this AO in bytecode, we now have an instruction
for it that simply invokes the C++ implementation.

This allows us to simplify Bytecode::Generator quite a bit by removing
all the variable scope tracking.

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -17,6 +17,7 @@
     O(BitwiseNot)                    \
     O(BitwiseOr)                     \
     O(BitwiseXor)                    \
+    O(BlockDeclarationInstantiation) \
     O(Call)                          \
     O(ConcatString)                  \
     O(ContinuePendingUnwind)         \