yams #72184 __hashmode__ hack seems useless and potentially dangerous [validation pending]

yams adds a __hashmode__ attribute for schemas that gets modified by cubicweb and checked in the __hash__ method, potentially changing the hash for a schema. That code seems to exist since 2006 but doesn't seem necessary with python >= 2.4 (or the tests don't show why).

done in0.34.0
load left0.000
closed by#4b2f4e5ad2a0 Stop setting/using the __hashmode__ attribute on schemas