Fold FindConversion() into FindConversionByName() and remove ACL check.

Fold FindConversion() into FindConversionByName() and remove ACL check.

All callers of FindConversionByName() already do suitable permissions
checking already apart from this function, but this is not just dead
code removal: the unnecessary permissions check can actually lead to
spurious failures - there's no reason why inability to execute the
underlying function should prohibit renaming the conversion, for example.
(The error messages in these cases were also rather poor:
FindConversion would return InvalidOid, eventually leading to a complaint
that the conversion "did not exist", which was not correct.)

KaiGai Kohei