Re #6902. Remove a hard coded path.

The one used for linux will give the right answer in a more general way
and will avoid looking into an installed Mantid when running a locally
built one.
Re #6902. Remove unnecessary code from linux build.

We use system-installed sip & PyQt on linux.
Revert "Re #6902. Remove unnecessary code from linux build."

This appears to have had a bad effect on our fragile MantidPlot
python tests and I'm not inclined to figure out what's wrong
exactly (they still work locally for me).

This reverts commit 424a87b8fd794b5573ac709a527a02b291f9f604.
Merge remote-tracking branch 'origin/feature/6902_built_and_installed_on_osx'