Remove MappedCollection converter; deprecate @converter

Removed the collection converter used by the :class:`.MappedCollection`
class. This converter was used only to assert that the incoming dictionary
keys matched that of their corresponding objects, and only during a bulk set
operation.  The converter can interfere with a custom validator or
:meth:`.AttributeEvents.bulk_replace` listener that wants to convert
incoming values further.  The ``TypeError`` which would be raised by this
converter when an incoming key didn't match the value is removed; incoming
values during a bulk assignment will be keyed to their value-generated key,
and not the key that's explicitly present in the dictionary.

Overall, @converter is superseded by the
:meth:`.AttributeEvents.bulk_replace` event handler added as part of
:ticket:`3896`.

Fixes: #3604
Change-Id: Id0f7bd2cec938f5975eb2ab94df9ba5754dd43c3

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