journal: remove journal_file_object_keep/release functions

journal: remove journal_file_object_keep/release functions

The only user is sd_journal_enumerate_unique() and, as explained in
the previous commit (fed67c38e3 "journal: map objects to context set by
caller, not by actual object type"), the use of them there is now
superfluous. Let's remove them.

This reverts major parts of commits:
  ae97089d49 journal: fix access to munmapped memory in
             sd_journal_enumerate_unique
  06cc69d44c sd-journal: fix sd_journal_enumerate_unique skipping values

Tested with an "--enable-debug" build and "journalctl --list-boots".
It gives the expected number of results. Additionally, if I then revert
the previous commit ("journal: map objects to context set by caller, not
to actual object type"), it crashes with SIGSEGV, as expected.