dont warn for dictionary passed positionally

dont warn for dictionary passed positionally

Fixed issue where use of the :paramref:`_sql.case.whens` parameter passing
a dictionary positionally and not as a keyword argument would emit a 2.0
deprecation warning, referring to the deprecation of passing a list
positionally. The dictionary format of "whens", passed positionally, is
still supported and was accidentally marked as deprecated.

Removes warning filter for case statement.

Fixes: #6786
Change-Id: I8efd1882563773bec89ae5e34f0dfede77fc4683