Dedup apiFile and removedApiFile fields

Previously, there were two separate fields for each of the API and
removed API files because in some cases it needed a WritablePath and in
other cases it could only be a Path.

This change refactors the code so that the WritablePath is no longer
needed, the duplicate fields deleted and the remaining field is changed
to Path. It also removes a few unused fields.

Bug: 234113632
Test: m nothing
Change-Id: If81f5fe2aa05d9d7b2bc23869fdb2397649d67b3