panel: remove deprecated methods: addClass, removeClass, and toggleClass

[bradrich]

Full deprecation of addClass, removeClass, and toggleClass is scheduled for the Material 1.2.0 release.
During that release, the current functionality of those methods will be removed and replace with an error.

[Splaktar]

The decision, rationale, and explanation of why this needs to happen can be found here:
#9231 (comment)