Aggregations: Remove `ordinals` execution hint.

This was how terms aggregations managed to not be too slow initially by caching
reads into the terms dictionary using ordinals. However, this doesn't behave
nicely on high-cardinality fields since the reads into the terms dict are
random and this execution mode loads all unique terms into memory.

The `global_ordinals` execution mode (default since 1.2) is expected to be
better in all cases.

Close #6499