Prune unused layers from the CrossTileSymbolIndex

[ChrisLoer]

Fixes issue #5995 (index leaked removed layers).

I don't think #5995 was a very big deal, but I just wanted to get this loose end out of the way. There are two changes here:

1. If you add and remove many layers with unique IDs, you won't end up with left-behind cruft in the CrossTileSymbolIndex
2. If you and and remove a layer with the same ID, you won't pick up the crossTileIDs from previously indexed versions of the layer. I don't think this change matters, because (a) crossTileIDs are always contained to one layer, and the entire layer was removed, so (b) there's no existing fade/collision state that's going to get copied over because you're using old IDs vs new IDs.

I added a basic unit test for the remove process and manually tested removing a layer from an active map and then adding it back.

/cc @ansis @mollymerp

[ansis]

looks good!

[mollymerp]

LGTM! (but shouldn't it be layerIndices 😛 )