Fix joinConnectedFeatures not keeping the featureParts list in sync

The method `Layer::joinConnectedFeatures()`, when merging connected features, only removes the obsolete features (i.e. those whose geometry was merged with another one) from the `rtree`, but not from the `featureParts` list. This causes `Layer::chopFeatures` to misbehave, and results in randomish behaviour when labeling multi-part lines with merge connected features activated.

Funded by Sourcepole.