Deprecate/remove MTI feature compiler?

Google's Noto project was almost certainly the only user of the MTI feature compiler (compiling Monotype FontDame text-format GSUB/GPOS dumps to binary). With all of the MTI format files now converted to FEA, this is no longer a requirement. While there's no harm having the old code around, and possibly some benefit in terms of separation-of-concerns in having two feature compiler subclasses, less code is cleaner and more maintainable than more code. We should deprecate and remove it.

No files, tests, or entry points are named. Search the repository for the MTI feature compiler and its references, inspect related tests and documentation, then confirm that the converted MTI format files no longer depend on it before removing or deprecating it and running the affected test suite.

Written by the indexing model from the issue text.