#342 alter table error on TW 1.8 DB upgrade

[Jams H Thompson]

(mysql): ALTER TABLE tiki_user_watches DROP COLUMN
`user` 1062: Duplicate entry
'wiki_page_changed-Asterisk' for key 1

adodb_mysql._execute(ALTER TABLE
tiki_user_watches DROP COLUMN `user`, false) % line
833, file: adodb.inc.php
adodb_mysql.execute(ALTER TABLE
tiki_user_watches DROP COLUMN `user`, null) % line
348, file: TikiDB.php
tikidb.query(ALTER TABLE tiki_user_watches DROP
COLUMN `user`, null, -1, -1, -1) % line 188, file:
TikiBase.php
tikiinstaller.query(ALTER TABLE tiki_user_watches
DROP COLUMN `user`) % line 186, file: TikiInstaller.php
tikiinstaller.upgradepackage(users) % line 42, file:
upgrade_packages.php