pgsql: Remove typeidIsValid() checks in can_coerce_type().

Log Message:
-----------
Remove typeidIsValid() checks in can_coerce_type().  These checks
were pretty expensive and I believe the case they were put in to
defend against can no longer arise, now that we have dependency checks
to prevent deletion of a type entry that is still referenced.  Certainly
the example given in the CVS log entry can't happen anymore.
Since this was the only use of typeidIsValid(), remove the routine too.

Modified Files:
--------------
    pgsql/src/backend/parser:
        parse_coerce.c (r2.128 -> r2.129)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_coerce.c.diff?r1=2.128&r2=2.129)
        parse_type.c (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/parse_type.c.diff?r1=1.74&r2=1.75)
    pgsql/src/include/parser:
        parse_type.h (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/parser/parse_type.h.diff?r1=1.29&r2=1.30)