Apple build: Prevent supernova from being installed twice

Currently binaries (including supernova) land in Content/MacOS as defined here: https://github.com/supercollider/supercollider/blob/qt52/editors/sc-ide/CMakeLists.txt

Removed lines above want to install supernova again in the old location and thus create an install-error.
Merge pull request #1256 from bagong/patch-5

Apple build: Prevent supernova from being installed twice