Yii2: [2.1] Finally remove deprecated stuff

[rugabarbo]

Roadmap Wiki section: https://github.com/yiisoft/yii2/wiki/Roadmap#cleanup – (BTW: will be good to link it with this issue).

For each code marked as @deprecated need to do:

• rewrite dependencies
• remove documentation occurrences
• remove code

Аlready done:

• 800574871419b99442d8be73a51e7f8e9e0d090f
• 4e15d3375633ae92eb0e91fbb0f6050de61b5d5c

[samdark]

Linked to this issue from wiki.

[rugabarbo]

All deprecations removed (last PR #15963 is waiting for the @SilverFire review). Some things can reappear after merges (2.0.x >> 2.1.x). That's why I'll wait for other 2.1 commits (and merges), then will search deprecated things again.

Don't close this task.

[SilverFire]

Merged #15963