bash-completion: Remove `admin` completions

bash-completion: Remove `admin` completions

The `admin` commandline should be considered a demo; I just added
the `pin` command *mostly* so we could use it for unit tests, although
I can imagine other people using it.

But maintaining completions is a lot of overhead right now, let's not
do it for `admin`.

The other command line options that operate on repos we will definitely maintain
since they're used in releng contexts.

Closes: #1468
Approved by: jlebon