It seems to me that all use cases for turning off the config are
covered by `hg rebase --keep`, `hg graft`, or `hg touch`. The use
cases I can think of are:
1. Create a temporary copy of a set of commits for some experimentation (use `hg rebase --keep` or `hg graft`).
2. Revive a pruned commit (use `hg touch`, possibly followed by `hg rebase` if you want to move it elsewhere).
Also, it is a config option after all, which means that it is not
intended for one-off `hg rebase` commands (then it would have been a
command-line flag). So, let's drop support for this config since it
seems unused. It will make my work on making `rewriteutil.precheck()`
check for divergence slightly simpler.