Remove references to MAKE_REMOTE_JOBS from CMakeLists.txt

Note: In order to build successfully, the branch with this commit must
also contain commit 3ec66fa4a0e9c8f342e1407ec3fe05c8a4995204, which was
part of ticket 7978.

Refs #7979
Remove cruft from the MantidPlot CMakeLists.txt file

The CMakeLists.txt file had some cruft left over from the original
remote job submission GUI work.  The .cpp and .h files were long gone,
but they were still mentioned in the file.

Refs #7978
Merge remote branch 'origin/feature/7979_RemoteJobConditionalBuild'