perf(bash-v2): short-circuit descriptionless candidate lists (#1686)

If the list of candidates has no descriptions, short circuit all the
description processing logic, basically just do a `compgen -W` for the
whole list and be done with it.

We could conceivably do some optimizations like this and more when
generating the completions with `--no-descriptions` in Go code, by
omitting some parts we know won't be needed, or doing some things
differently. But doing it this way in bash, the improvements are
available also to completions generated with descriptions enabled when
they are invoked for completion cases that produce no
descriptions. The result after this for descriptionless entries seems
fast enough so it seems there's no immediate need to look into doing
that.