remove deprecated ipyparallel methods now that they are broken anyway

[minrk]

no version of ipython parallel without these methods will work with ipykernel 7 anyway, so might as well remove them since they can never be used.

Restores should_handle to be sync in the default implementation (it only returns True now that the ipyparallel code is removed), which was an unnecessary breakage. It is allowed to be async in subclasses.

alternative to the updated deprecation messages in #1272