access-om2 qa checks: Remove option for different forcing keywords

[aidanheerdegen]

We currently allow any of the following forcing mode keywords

'forcing mode': {'repeat-year', 'ryf', 'repeat-decade', 'rdf', 'interannual', 'iaf'}

It is just confusing and leads to a lack of consistency.

Either we should remove the ryf, rdf and iaf options, or insist on both for each forcing mode, and make each set mutually exclusive.

e.g. something like so

'forcing mode': {{'repeat-year', 'ryf'}, 
                 {'repeat-decade', 'rdf'},
                 {'interannual', 'iaf'}}

But it would require rewriting the logic of that test.