Finally remove all backends desktopfiles
This is the last step for porting cirkuit's plaugins to the kf5
framework. The included json metadata files have been converted with the
tool desktop_to_json.
Improvements:
- desktopfiles are not converted to json format each time the project
is build.
- circuit_backend.desktop does not need to be installed anymore.
- kcoreaddons_desktop_to_json() does not complain about missing
circuit_backend.desktop, if you try to build cirkuit without an old
version being already installed in the system.