effects: Strip unused builtin effect metadata

This improves plugin loading times. As is, the main issue is the number
of builtin effects and the fact that each has a lot of translated
strings, which combined adds up to noticeable loading times. KWin itself
will never read those translated strings, it only needs two pieces - the
plugin id and whether the plugin is enabled by default.

This change adds a little helper to strip unnecessary info from metadata
files.