Refs #10838. Suppress new clang warnings. Fix remaining warning.

Additional warning options were added to the osx+clang build in ticket #10685. This is to temporarily suppress these warnings on OSX. I also removed an unused private field that was still generating a warning.

testing: given the minimal changes, code review plus checking the build results should be sufficient.

http://trac.mantidproject.org/mantid/ticket/10838