UP-4289 Clean up IPerson

Clean up `IPerson` in non-backwards-compatible ways ([UP-4289](https://issues.jasig.org/browse/UP-4289)).

Under current versioning policy appropriate for minor release `4.2` (not backwards compatible, but inflicts only modest upgrade pain).  Under Semantic Versioning would be appropriate only on a MAJOR release (breaks APIs).  Could choose to hold this for uPortal 5 anyway since it is touching an important API.  However, I don't think it's actually elminating any methods that are needed or likely to have been used in local code.

Eliminates `IPerson` method `getAttributes()`.  This method is unused in the uPortal codebase.  It seems to return the user attribute values out of context of their keys, which doesn't seem useful.  To the extent that anyone did need that ability, can get that from the attribute map value `values()`, which is what the only nontrivial implementation of this method did anyway.

Eliminates `IPerson` method `getAttributeNames()`.  Use `getAttributeMap().keySet()` instead.  Refactors the one usage of `getAttributeNames()` to that approach.

Eliminates `IPerson` method `setEntityIdentifier()`. (Unused).

Fixes a couple trivial JavaDoc annoyances.