fix a bug that prevents launch-time passphrases w/ cred plugins

with the advent of credential plugins there's no way for us to *actually
know* the RSA key value at the time the credential is _created_, because
the order of operations is:

1.  Create the credential with a specified passphrase
2.  Associate a new dynamic inventory source pointed at some third party
    provider (hashi, cyberark, etc...)

this commit removes the code that warns you about an extraneous
passphrase (if you don't specify a private key)

additionally, the code for determining whether or not a credential
_requires_ a password/phrase at launch time has been updated to test
private key validity based on the *actual* value from the third party
provider

see: https://github.com/ansible/awx/issues/4791

awx/main/tests/functional/test_credential.py

@@ -137,7 +137,6 @@ def test_credential_creation(organization_factory):
     [PKCS8_PRIVATE_KEY, None, True],  # unencrypted PKCS8 key, no unlock pass
     [PKCS8_PRIVATE_KEY, 'passme', False],  # unencrypted PKCS8 key, unlock pass
     [None, None, True],  # no key, no unlock pass
-    [None, 'super-secret', False],  # no key, unlock pass
     ['INVALID-KEY-DATA', None, False],  # invalid key data
     [EXAMPLE_PRIVATE_KEY.replace('=', '\u003d'), None, True],  # automatically fix JSON-encoded GCE keys
 ])