obsolete the DeleteCollection method

fixes #73

For a more complete fix, we should remove this in some future major revision.

Note:

This doesn't impact performance at all. The `DeleteCollection` method was simply a shortcut for querying and deleting entities.

The main reason for this is to make the behavior of `Save` easier to reason about. `Save` now deals exclusively with write operations -- this was an odd case where reads were executed during a save.