pylint: Fix unused-private-member

Pylint 2.9.0 introduced new checker:
> Emitted when a private member of a class is defined but not used

Fixes: https://pagure.io/freeipa/issue/9117
Signed-off-by: Stanislav Levin <slev@altlinux.org>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>

ipaserver/install/custodiainstance.py

@@ -303,9 +303,3 @@ class CustodiaInstance(SimpleServiceInstance):
         data = {'prefix': 'ca',
                 'list': certlist}
         self._get_keys(cacerts_file, cacerts_pwd, data)
-
-    def __start(self):
-        super(CustodiaInstance, self).__start()
-
-    def __enable(self):
-        super(CustodiaInstance, self).__enable()