Remove X-KWin-Exclude-Listing metadata flag

We only have one KWin script making use of this and that one is shown
regardless. The issue with this flag is that it will hide scripts from
the user without a possibility to see them.

For effects, we have quite a few internal ones and a possibility to show
even hidden ones. But we can not reuse this logic for the scripts.

BUG: 458572
FIXED-IN: 6.0