[3.32] fix(reledit): Do not retrieve a list of schemata with _compute_ttypes
It now uses the yams.schema.RelationSchema.objects method with an etype parameter. Without this parameter, this method returns all the schemata which can be subject of the specified relation.
Removed "except KeyError" which was obsoleted by this change.
--HG-- branch : 3.32
backport !354 (merged)