Remove old admin API `GET /_synapse/admin/v1/users/<user_id>` (#9401)

Related: #8334
Deprecated in: #9429 - Synapse 1.28.0 (2021-02-25)

`GET /_synapse/admin/v1/users/<user_id>` has no
- unit tests
- documentation

API in v2 is available (#5925 - 12/2019, v1.7.0).
API is misleading. It expects `user_id` and returns a list of all users.

Signed-off-by: Dirk Klimpel <dirk@klimpel.org>