restapi: Disable Get Groups For Non-Admin User

Getting Groups for non-admin user is wrongly implemented. It currently returns -
instead of all groups which the user is authorized to see - exactly one User entity,
(the user himself). Needless to say this is very far from the intended result.

Until new implementation will be avaliable, I am removing this wrong implementation.
For now the outcome will now be: user will fail due to insufficient permission.

Change-Id: I932a197271ccfa52418276ec4ddfb4c5db95e0a0
Signed-off-by: Ori Liel <oliel@redhat.com>