pgsql: Remove "convert 'blah' using conversion_name" facility, because

Log Message:
-----------
Remove "convert 'blah' using conversion_name" facility, because if it
produces text it is an encoding hole and if not it's incompatible
with the spec, whatever the spec means (which we're not sure about anyway).

Modified Files:
--------------
pgsql/doc/src/sgml:
func.sgml (r1.397 -> r1.398)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml?r1=1.397&r2=1.398)
pgsql/doc/src/sgml/ref:
create_conversion.sgml (r1.19 -> r1.20)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/create_conversion.sgml?r1=1.19&r2=1.20)
pgsql/src/backend/catalog:
pg_conversion.c (r1.37 -> r1.38)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/pg_conversion.c?r1=1.37&r2=1.38)
pgsql/src/backend/parser:
gram.y (r2.602 -> r2.603)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/gram.y?r1=2.602&r2=2.603)
keywords.c (r1.191 -> r1.192)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/keywords.c?r1=1.191&r2=1.192)
pgsql/src/include/catalog:
pg_proc.h (r1.471 -> r1.472)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h?r1=1.471&r2=1.472)
pgsql/src/include/utils:
builtins.h (r1.303 -> r1.304)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/builtins.h?r1=1.303&r2=1.304)
pgsql/src/test/regress/expected:
conversion.out (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/conversion.out?r1=1.13&r2=1.14)
pgsql/src/test/regress/sql:
conversion.sql (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/conversion.sql?r1=1.10&r2=1.11)