Remove OFP frontend

[mlange05]

The OFP frontend is getting increasingly hard to maintain. A recent brush with some of its inconsistencies (mostly due to skew in the generated AST node scoping due to upstream bugs) has highlighted that in particular better handling of functions/procedures will become very painful if we are to maintain this.

I've checked and the most recent release (0.6.1) is not compatible with our frontend implementation, but it does still have many of the scoping quirks that we tend to bend over backwards to circumvent. As we have little use for it that we cannot deal with via other frontends, I propose we now fully deprecate and/or entirely remove the OFP frontend. This should improve test throughput and remove some of the special-casing, ideally before we continue to align procedures as proper IR nodes.

I'm happy to implement and test this, but input and feedback are very welcome (@reuterbal ).