Remove unused method basic_auth from rest client

This method is not being used. Removing to reduce the area we need to cover in
the unit tests.

Related to bp unit-tests

Change-Id: I4ede8975d6fbbd4e5a4366f1ae2d771ffa526f91
diff --git a/tempest/common/rest_client.py b/tempest/common/rest_client.py
index 3efc710..966c277 100644
--- a/tempest/common/rest_client.py
+++ b/tempest/common/rest_client.py
@@ -126,21 +126,6 @@
 
         return self.token
 
-    def basic_auth(self, user, password, auth_url):
-        """
-        Provides authentication for the target API.
-        """
-
-        params = {}
-        params['headers'] = {'User-Agent': 'Test-Client', 'X-Auth-User': user,
-                             'X-Auth-Key': password}
-
-        resp, body = self.http_obj.request(auth_url, 'GET', **params)
-        try:
-            return resp['x-auth-token'], resp['x-server-management-url']
-        except Exception:
-            raise
-
     def keystone_auth(self, user, password, auth_url, service, tenant_name):
         """
         Provides authentication via Keystone using v2 identity API.