Remove (almost) non-functional and deprecated RMS env settings

Backwards compatibility probably means that it exists in user
configurations, but Ert will not do anything if a RMS forward model is
configured with an unused option, like:

  FORWARD_MODEL RMS(..., <RMS_SOMETHING_UNRECOGNIZED>=FOOBAR)

so it should be safe to remove altogether.

When present, these two variables yield two warnings emitted pr.
realization due to the variables not being defined.

Author: berland

src/runrms/_forward_model.py

@@ -66,19 +66,6 @@ def documentation() -> ForwardModelStepDocumentation | None:
         )
 
 
-@plugin(name="runrms")  # type: ignore
-def forward_model_configuration() -> dict[str, dict[str, str]]:
-    """These exist for backward compatibility.
-
-    If `RMS_PYTHONPATH` is set to a non-existing path it can fail."""
-    return {
-        Rms().name: {
-            "RMS_PYTHONPATH": "<RMS_PYTHONPATH>",
-            "RMS_PATH_PREFIX": "<RMS_PATH_PREFIX>",
-        }
-    }
-
-
 @plugin(name="runrms")  # type: ignore
 def installable_forward_model_steps() -> list[ForwardModelStepPlugin]:
     return [Rms]