Fixed migration error

Migration error occurs when uninstall redmine_blogs.
I have removed remove_index below drop_table in db/migrate/001_create_blogs.rb.

```
$ rake db:migrate:plugin NAME=redmine_blogs VERSION=0 RAILS_ENV=production
(in /var/www/redmine)
Migrating redmine_blogs to version 0 ...
==  ActsAsTaggable: reverting =================================================
-- drop_table(:taggings)
   -> 0.0300s
-- drop_table(:tags)
   -> 0.0201s
==  ActsAsTaggable: reverted (0.0502s) ========================================

==  FixBlogsDescriptionType: reverting ========================================
==  FixBlogsDescriptionType: reverted (0.0000s) ===============================

==  CreateBlogs: reverting ====================================================
-- drop_table(:blogs)
   -> 0.0243s
-- remove_index(:blogs, :project_id)
rake aborted!
An error has occurred, all later migrations canceled:

Mysql::Error: Table 'redmine.blogs' doesn't exist: SHOW KEYS FROM `blogs`

(See full trace by running task with --trace)
```