Cleanup \OCP\Share

[rullzer]

Now that the access list is converted as well we should make work of cleaning up the old sharing code properly.

• Get rid of the registerbackend stuff (we have the factory for that, or we should add functions to the manager to check this)
• Get rid of all calls to \OCP\Share::
  • mainly in test code
  • maybe to this in small steps for easier review
• Kill \OC\Share\

[brantje]

Please notify developers before removing it. I already had a breaking change on a live app.

[szaimen]

I suppose this issue is still valid? If not, please close this issue!