MB-20623: MutationLog: Remove unused Delete and Uncommitted functionality

MB-20623: MutationLog: Remove unused Delete and Uncommitted functionality

The MutationLog (when actually used for Mutations) could record
Deletes (in addition to 'adds') of keys, and return a list of
'uncommitted' keys.  However this functionality has long been unused
(since March 2013, http://review.couchbase.org/26943).

Remove this unused code to simplify the way the MutationLog class is
now used - for the access log.

Change-Id: I5642553708c7d5cf320cec5524ae49643a414192
Reviewed-on: http://review.couchbase.org/68041
Reviewed-by: Daniel Owen <owend@couchbase.com>
Tested-by: buildbot <build@couchbase.com>