JO shouldn't hardcode ATHENA_PROC_NUMBER

The environment variable for multi-threading ATHENA_PROC_NUMBER should be set by prodsys, not the JOs.

Can we make the CI fail if the JOs try to assign a value to that? (The JO are free to ask if this environment variable exists and what it's value is (e.g. to pass it into Madgraph), but they shouldn't try to overwrite its value

See e.g. MR !745 (merged) where this had to be corrected, but e.g. this JO where it's used in an acceptable way.