remove WSS and AWI as metrics

Metrics such as WSS/AWI require choosing on a reference to compare the predictions score against. While for ROCSS or BSS the reference is well defined analytically. In evalhyd, the reference for WSS/AWI is taken as the "climatology". Since the true climatology cannot be determined, it has to be approximated on a given period. But the period does not have to be the sample period, and could be a much longer, possibly independent, period. Also other references could be chosen altogether, e.g. the persistence.

In addition, to be consistent, other skill scores should be provided, e.g. CRPSS, QSS, etc. But the choice of a reference would remain.

So it is better to remove WSS/AWI from the list of metrics in evalhyd, and only keep those where the reference is well defined analytically.