clients/v1: Remove DeleteUser RPC from UsersService (#98)

part of [CORE-1195: SAMS: remove DeleterUser endpoint from clientsv1,
and user management
RPC](https://linear.app/sourcegraph/issue/CORE-1195/sams-remove-deleteruser-endpoint-from-clientsv1-and-user-management),
and addressing @unknwon's
[comment](sourcegraph/sourcegraph-accounts#376 (review))

We initially implemented the `DeleteUser` RPC to handle all CRUD user
management actions in the Enterprise Portal, but since then we have
decided not to implement any deletion functionality in Enterprise
Portal. To clean up the UsersService API, removing this to avoid
managing 2 separate DeleteUser implementations.

Thanks to
[DeepSearch](https://sourcegraph.sourcegraph.com/deepsearch/fab7f45b-2772-4346-ab74-ecb6af7ccc3c),
this should be safe to remove as there are no downstream calls to the
`DeleteUser` RPC


## Test plan
CI

Author: kalanchan