git-p4: remove "rollback" verb

git-p4: remove "rollback" verb

The "rollback" verb implements a simple algorithm which takes the set of
remote perforce tracker branches, or optionally, the complete collection
of local branches in a git repository, and deletes commits from these
branches until there are no commits left with a perforce change number
greater than than a user-specified change number. If the base of a git
branch has a newer change number than the user-specified maximum, then
the branch is deleted.

In future, there might be an argument for the addition of some kind of
"reset this branch back to a given perforce change number" verb for
git-p4. However, in its current form it is unlikely to be useful to
users for the following reasons:

  * The verb is completely undocumented. The only description provided
    contains the following text: "A tool to debug the multi-branch
    import. Don't use :)".

  * The verb has a very narrow purpose in that it applies the rollback
    operation to fixed sets of branches - either all remote p4 branches,
    or all local branches. There is no way for users to specify branches
    with more granularity, for example, allowing users to specify a
    single branch or a set of branches. The utility of the current
    implementation is therefore a niche within a niche.

Given these shortcomings, this patch removes the verb from git-p4.

Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>