Remove AutoCloseable from internal PackFile and friends

Remove AutoCloseable from internal PackFile and friends

PackFile is held by the block cache and cannot be auto closed in a
try-with-resources statement.  Remove the interface as JGit does
explicit management of the instances.

ObjectDatabase and RefDatabase are internal details of Repository
and are managed with the Repository. Marking them AutoCloseable
provides no value to the library or an application using the API.

Change-Id: Ibee19eadd66233e6666b601583daa1834a7778f1