LibJS: Remove unused InstructionStreamIterator::source_code getter

Author: https://github.com/ADKaster
Commit: 59b1fb23a9
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4362
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/Hendiadyoin1

Libraries/LibJS/Bytecode/Instruction.cpp

@@ -105,11 +105,6 @@ UnrealizedSourceRange InstructionStreamIterator::source_range() const
     };
 }
 
-RefPtr<SourceCode> InstructionStreamIterator::source_code() const
-{
-    return m_executable ? m_executable->source_code.ptr() : nullptr;
-}
-
 Operand::Operand(Register reg)
     : m_type(Type::Register)
     , m_index(reg.index())