lib/config: Deprecate commit-update-summary option

lib/config: Deprecate commit-update-summary option

Now that we have `auto-update-summary`, there is no point in having
`commit-update-summary`. The latter also only had an effect through
the `commit` CLI command, whereas the former is embedded directly in
libostree.

There is one corner case that slips through: `commit` would update the
summary file even if orphan commits were created, which we no longer do
here. I can't imagine anyone relying on this, so it seems safe to drop.

Closes: #1689
Closes: #1693
Approved by: mwleeds