[lldb-mi] Remove MIUtilSystem*.
This platform-specific code wasn't fully implemented and wasn't
actually needed. There was one call for the log file path and
that has been addressed.
This lets us also remove an error message from MICmnLogMediumFile
as it is no longer used.
Reviewers: ki.stfu, domipheus, abidh
Differential Revision: http://reviews.llvm.org/D12764