Further work would be nice to be able to only collect zones that
probably have garbage to collect, but that likely requires splitting our
contexts in more zones.
0 A.D. had been running in incremental GC since 1.8.5. With the SM78
upgrade, I changed this to the new default "ZONE_INCREMENTAL", which GCs
specific zones.
However, I failed to realise that deleted scriptInterfaces and their
corresponding zones would no longer be collected.
This corrects for that.
Fixes d92a2118b0.
Refs #5861
Differential Revision: https://code.wildfiregames.com/D3220
This was SVN commit r24392.