Code cleanup in Canvas2DLayerManager

Deleted private Canvas2DLayerManager::removeLayerFromList which was never used (and only comported asserts)

Review URL: https://codereview.chromium.org/331383004

git-svn-id: svn://svn.chromium.org/blink/trunk@176657 bbb929c8-8fbe-4397-9dbb-9b2b20218538