ostree/config: Add an "unset" operation

ostree/config: Add an "unset" operation

Currently there's a way to set a key to the empty string but there's no
way to unset it completely (remove the key from the group). This might
be helpful for instance if you want to temporarily set
"core.lock-timeout-secs" to a specific value for the duration of one
operation and then return it to the default after that operation
completes.

This commit implements an "unset" operation for the config command, adds
a unit test, and updates the man page.

Closes: #1743
Approved by: cgwalters