Remove an optimisation that tracks whether or not a thin metadata commit
is needed.

If dm_pool_commit_metadata() is called and no changes have been made
to the metadata then this optimisation avoided writing to disk.

Removing because we're going to do something better later.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>