pgsql: Remove obsolete internal functions istrue, isfalse, isnottrue,

Log Message:
-----------
Remove obsolete internal functions istrue, isfalse, isnottrue, isnotfalse,
nullvalue, nonvalue.  A long time ago, these were used to implement the SQL
constructs IS TRUE, etc.

Modified Files:
--------------
    pgsql/src/backend/utils/adt:
        bool.c (r1.43 -> r1.44)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/bool.c?r1=1.43&r2=1.44)
        misc.c (r1.63 -> r1.64)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/misc.c?r1=1.63&r2=1.64)
    pgsql/src/include/catalog:
        catversion.h (r1.492 -> r1.493)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.492&r2=1.493)
        pg_proc.h (r1.516 -> r1.517)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.516&r2=1.517)
    pgsql/src/include/utils:
        builtins.h (r1.321 -> r1.322)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.321&r2=1.322)