I have Ubuntu 17.04.
I have Tarantool 1.7, pulled from source yesterday.
When I try to drop the role 'super', I succeed.
When I try to drop the user 'guest', I fail.
Example:

tarantool> box.schema.role.drop('super')

...

tarantool> box.schema.user.drop('guest')

• error: 'Failed to drop user or role ''guest'': the user or the role is a system'
  ...