remote: 'show' and 'prune' can take more than one remote

remote: 'show' and 'prune' can take more than one remote

The 'git remote show' and 'prune' subcommands are documented as taking
only a single remote name argument, but that is not the case; they
will simply iterate the action over all remotes given.  Update the
documentation and tests to match.

With the last user of the -f flag gone, we also remove the code
supporting it.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>