Remove redundant Collection stub

[jaikdean]

Doctrine\Common\Collections\Collection has Psalm annotations as of 1.6.0. One or two of the annotations in the stub were incorrect, and are fixed in doctrine/collections.

[weirdan]

Not going to merge this

1. doctrine/collection currently has wrong type for Collection::key()
2. People may still use 1.0-1.5

What I'm going to do instead is fix the wrong types for partition (I suppose you were referring to them), do a PR to doctrine/collections correcting the key() type and once that fixed, selectively disable Collection stub when running with doctrine/collections 1.6.X+

[weirdan]

key() type was fixed in doctrine/collections@6aab75b (not released yet)