tests: drop dhparam default length configuration

This has been the default value since the option was introduced back in
2018[0].

[0] 81fc2c3509

tests/lib/config.nix

@@ -1,7 +1,4 @@
 {
   # Testing eval failures that result from stateVersion assertion is out of scope
   mailserver.stateVersion = 999;
-
-  # minimum size required by dovecot
-  security.dhparams.defaultBitSize = 2048;
 }