Deleting annotation from an action.

[Dominic Kim <st...@gmail.com>]

Dear all.

There are open PRs to introduce a feature to delete existing annotations
from an action.
Currently, we can update values of annotations but we can't explicitly
delete annotations via CLI.

This change will introduce a flag to list up annotations to be deleted from
the given action.

This is the main PR in the core repo:
https://github.com/apache/openwhisk/pull/4940

and these are its counterparts.
https://github.com/apache/openwhisk-cli/pull/488
https://github.com/apache/openwhisk-client-go/pull/137
<https://github.com/apache/openwhisk/pull/4940>
Any feedback would be appreciated.
I would merge them next week if there is no other feedback.

Thanks.
Dominic.