That was never actually finished and we do not need to keep it around anymore.
Whether we add its successor once that is ready for production use remains to be
seen.
That is, most of the business code should not be aware that it is being compiled
to multiple versions even when it heavily interacts with MC, preprocessor
statements should be an escape hatch, not the norm.
Similarly, code should not be forced to do `MCVer.getWindow(mc)` instead of the
much more intuitive `mc.getWindow()`, and a new preprocessor (technically remap)
feature makes this possible by defining "search and replace"-like patterns (but
smarter in that they are type-aware) in one or more central places (the
"Patterns.java" files) which then are applied all over the code base.
In a way, this is another step in the automatic back-porting process where
preprocessor statements are used when we cannot yet do something automatically.
Previously we "merely" automatically converted between different mapping, this
new feature now also allows us to automatically perform simple refactoring
tasks like changing field access to a getter+setter (e.g. `mc.getWindow()`), or
changing how a method is called (e.g. `BufferBuilder.begin`), or changing a
method call chain (e.g. `dispatcher.camera.getYaw()`), or most other
search-and-replace-like changes and any combination of those.
The only major limitation is that the replacement itself is not smart, so
arguments must be kept in same order (or be temporarily assigned to local
variables which then can be used in any order).
Removes the need for //#if statements on each and every event handler.
Instead we now always use the equivalent Callback and have one central 1.12.2
class which forwards Forge events to the callbacks.
