Straighten removing strategy from collection field

[malarzm]

Fixes #1174. This commit finishes removing "unused" strategy from collection field. People who are still using strategy combined with collection and annotation mapping will now start getting error because AbstractField no longer has that property (which was the reason why our tests were green but YML/XML mappings suffered as reported in #1174)

@redthor could you maybe run your test suite against this PR?

[redthor]

The test still fails but neither driver contains strategy:

1) Doctrine\ODM\MongoDB\Tests\Functional\Ticket\GH1174Test::compareDrivers
Failed asserting that two arrays are equal.
--- Expected
+++ Actual
@@ @@
     'isInverseSide' => false
+    'options' => Array ()
+    'value' => null
 )

I then checked out your branch in our dev code and it appears to be all working.

So my test case is perhaps not something we need?

Thanks for looking into this.

[malarzm]

So my test case is perhaps not something we need?

I added this assert: https://github.com/doctrine/mongodb-odm/pull/1177/files#diff-6a4e49bb800973ef639a55d44888ff43R50 I'd say the rest is out of scope of this PR. But anyway thank you very much for that test, I would spend a lot more time looking for the culprit if not for your test :)

[jmikola]

LGTM. I don't have any recollection of strategy being used for the collection field type. Both it and hash are treated like basic fields where any changes are persisted with $set. The collection strategies only apply when dealing with EmbedMany and ReferenceMany relationships.