feat: do not resolve MX records during configuration

MX record lookup was only used to detect Google Workspace domains.
They can still be configured manually.
We anyway do not want to encourage creating new profiles
with Google Workspace as we don't have Gmail OAUTH2 token anymore
and new users can more easily onboard with a chatmail relay.

deltachat-rpc-client/tests/test_something.py

@@ -570,8 +570,11 @@ def test_provider_info(rpc) -> None:
     assert provider_info is None
 
     # Test MX record resolution.
+    # This previously resulted in Gmail provider
+    # because MX record pointed to google.com domain,
+    # but MX record resolution has been removed.
     provider_info = rpc.get_provider_info(account_id, "github.com")
-    assert provider_info["id"] == "gmail"
+    assert provider_info is None
 
     # Disable MX record resolution.
     rpc.set_config(account_id, "proxy_enabled", "1")