upgrade: rename 'removecldeltachain' to 'plain-cl-delta'
The new naming is more descriptive of a "state" while the older one was more
about "action". I'm looking into command exposing more of data about the state
of the repository so "state" oriented work better there.
The key has not been made public anywhere outside the debug area so it is fine
to update it.
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors