magit-rebase{,-interactive}: remove an old trick

Back in the days `magit-rebase' and `magit-rebase-interactive' were
invoked directly, not through the rebase popup.  So we had to make
sure the user did not try to begin rebasing when another sequence
was already in progress.  We don't have to do that anymore.
magit-rebase{,-onto,-interactive}: improve doc-strings
magit-bisect-{good,skip,bad}: move definitions to magit-bisect.el