Comment out the revert filter taggings migration to stop failures

If you set up otwarchive and run the migrations for the first time, they currently fail on the revert_filter_taggings migration. 
```
==  RevertFilterTaggingsPrimaryKey: migrating =================================
-- execute("ALTER TABLE `filter_taggings` DROP INDEX `primary`, ADD PRIMARY KEY (`id`);")
rake aborted!
An error has occurred, all later migrations canceled:

Mysql2::Error: Can't DROP 'primary'; check that column/key exists: ALTER TABLE `filter_taggings` DROP INDEX `primary`, ADD PRIMARY KEY (`id`);/Users/sarken/otwarchive/db/migrate/20130707160814_revert_filter_taggings_primary_key.rb:5:in `up'
Tasks: TOP => db:migrate
```
revert_filter_taggings was there to undo [filter_tagging_primary_key](https://github.com/otwcode/otwarchive/blob/master/db/migrate/20120131225520_filter_tagging_primary_key.rb), but filter_tagging_primary_key is already commented out, so there's nothing to drop.