Do not use "del os.environ" as the variable might not exist

The use of del os.environ assumes that the environment variable exists.
If the variable does not exist, this call will result in a traceback.
The solution is to use os.environ.pop(VARIABLE, None) instead.

This is the ansible-freeipa fix for https://pagure.io/freeipa/issue/9446
(Nightly test failure for replica installation with --setup-ca)