[LLDB] Remove lldb-mi

As discussed on the mailing list [1], this patch removes the lldb-mi
tool and its tests from the LLDB repository. We moved lldb-mi into a
separate repository on GitHub [2] for downstream users or maintainers to
build and package.

[1] http://lists.llvm.org/pipermail/lldb-dev/2019-July/015103.html
[2] https://github.com/lldb-tools/lldb-mi

Differential revision: https://reviews.llvm.org/D64255

llvm-svn: 366465