Deprecate two un-needed EntityCeDisplay methods

Problem/Motivation

It's unfortunate, but I already created some public methods in EntityCeDisplayInterface that are just unnecessary:

• setOriginalMode(): I never wanted to create this but thought it was necessary because we need to set a class property $originalMode that is protected, from 'outside'.
  • That was dumb, because ConfigEntityInterface::set() exists.
• getConfigDependencyEntityViewDisplays(): Near the end of #3475612: Config dependencies are not properly calculated (just before 3.0.0 had to be released) I argued for making this public because I wanted to reuse/call it from the form.
  • Reuse turned out to be impossible, per #3475342-4: Improve CE UI form options visibility and messages, so I made the method protected and removed it from the interface.

Proposed resolution

I would rather set a good example for the future, even though deprecating methods so early is weird. So:

• Remove both methods from the interface
• Mark the public methods as @deprecated. (In the case of getConfigDependencyEntityViewDisplays(), this means making it public again; the public-to-protected change was only commited to -dev so far.)

...because in principle, someone could be calling these two methods from another class.

Remaining tasks

Review MR.

Write short change record mentioning the 2 deprecations.