Remove guard against multiple flow threads

Such a guard should be local site configuration, not enforced for ert as
a whole.

Flow threads are lightweight IO helper threads, and local conditions
determine whether they should be used or not.

Author: berland

src/ert/plugins/hook_implementations/forward_model_steps.py

@@ -354,11 +354,6 @@ def validate_pre_experiment(self, fm_json: ForwardModelStepJSON) -> None:
                 "Do not supply --np as an option to FLOW, "
                 "set NUM_CPU in the Ert config instead."
             )
-        if "--threads" in self.private_args.get("<OPTS>", ""):
-            raise ForwardModelStepValidationError(
-                "Do not supply --threads as an option to FLOW, "
-                "set NUM_CPU in the Ert config instead."
-            )
 
         available_versions = _available_flow_versions(
             env_vars=fm_json["environment"] or {}