Remove our existing compiler flags since they were only being used for
enabling/disabling passes to aid debugging and to simplify in preparation for
the upcoming work on diagnostics and bailouts. Additionally with the new
playground tabs disabling passes has become less necessary. In the future when
we have actual compiler flags (eg tweaking optimization levels) we can add this
back.
I opted to keep the existing `CompilerResult` return value instead of just
returning the optimized AST as we're still using `scopes` in our test fixtures.