pgsql: Fold FindConversion() into FindConversionByName() and remove ACL

Log Message:
-----------
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

Modified Files:
--------------
    pgsql/src/backend/catalog:
        namespace.c (r1.121 -> r1.122)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/namespace.c?r1=1.121&r2=1.122)
        pg_conversion.c (r1.48 -> r1.49)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_conversion.c?r1=1.48&r2=1.49)
    pgsql/src/include/catalog:
        pg_conversion_fn.h (r1.5 -> r1.6)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_conversion_fn.h?r1=1.5&r2=1.6)