Problem/Motivation
Follow up to #2949964: Add an EntityOwnerTrait to standardize the base field needed by EntityOwnerInterface.
Because of a bug in our test coverage, the Comment entity requires that the default owner is the anonymous user for the tests to pass. In order to provide better API and REST support for automatically associating the logged in user with created comments, we should ensure new comments are owned by the logged in user by default.
Proposed resolution
Delete getDefaultEntityOwner, which requires no deprecation because it was introduced at the start of the 8.7.x cycle and has never made it into a stable release.