No longer group features by geometry type

With symbolizer literals now being geometry type specific, we no
longer need the overhead to query the RTree separately for each
geometry type and render symbolizer groups by geometry type.

The geometry type index of the FeatureCache is no longer needed.

The filtering functionality of the FeatureCache's
getFeaturesObject method can be removed because it is no longer
used.
Merge pull request #1089 from ahocevar/no-geometry-type-groups

No longer group features by geometry type