[lldb][NFC] Cleanup mentions and code related to lldb-mi
Summary: lldb-mi has been removed, but there are still a bunch of references in the code base. This patch removes all of them.
Reviewers: JDevlieghere, jfb
Reviewed By: JDevlieghere
Subscribers: dexonsmith, ki.stfu, mgorny, abidh, jfb, lldb-commits
Differential Revision: https://reviews.llvm.org/D64992