Removes dead code from trove/tests/api/root.py

Reason:
- ./trove/tests/api/root.py uses undefined method
  create_mysql_connection in log_in_as_root.
- log_in_as_root is not used anywhere in any tests.

Changes:
- Removed log_in_as_root method from ./trove/tests/api/root.py

Change-Id: Ia5ab6538cab54a98b8057d0bf4c39dda9c7e3bf8
Closes-Bug: #1261174
Merge "Removes dead code from trove/tests/api/root.py"