Use a python script to strip effect metadata

This makes it easier to cross-compile KWin since it is no longer necessary
to have all the KWin dependencies on the host machine. This could be
partially addressed by moving the strip-effects-metadata.cpp into a
separate folder than can be built as a top-level project, thereby reducing
the dependencies to just QtCore. However, it still means we have to build
a native binary. Since all this script is doing is removing some JSON keys,
we could also use a python script and avoid the need to compile a
build-time helper program.