Fix syntax errors in contrib uninstall scripts

Fix a number of syntax errors in contrib modules' uninstall scripts.
Most of the changes add the mandatory USING clause to DROP OPERATOR
CLASS statements.  DROP TYPE is now DROP TYPE CASCADE; without
CASCADE a DROP TYPE fails due to the circular dependency on the
type's I/O functions.  The DROP FUNCTION statements for the I/O
functions have been removed, as DROP TYPE CASCADE removes them
automatically.

To test the uninstall scripts I did the following:

1. Create a database.
2. Run pg_dump to get a "before" dump.
3. Run a module's install script.
4. Run a module's uninstall script.  Look for errors and drops
   that cascade to anything other than types' I/O functions.
5. Run pg_dump to get an "after" dump.
6. Diff the "before" and "after" dumps; they should be identical.

For past discussion see the following thread:

http://archives.postgresql.org/pgsql-hackers/2006-02/msg01265.php
http://archives.postgresql.org/pgsql-hackers/2006-03/msg00096.php

Absent much feedback I chose option (2) in the following message:

http://archives.postgresql.org/pgsql-hackers/2006-03/msg00354.php

It might be desirable to wrap the drops in a transaction as option
(3) discusses.  pg_trgm's uninstall script is currently the only
one that does so; it was already like that so I didn't change it.

--
Michael Fuhr