[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

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64992

llvm-svn: 366590