[lldb] Remove linux/mips debugging support

As discussed on lldb-dev
<https://lists.llvm.org/pipermail/lldb-dev/2021-March/016777.html> the
mips code is unmaintained and untested. It also carries a lot of
technical debt which is not limited to mips-specific code.

Generic mips support remains (and is going to be used by the upcoming
freebsd code). Resurrecting mips support should be a matter of re-adding
the relevant register context files (while avoiding reintroducing the
debt).