Fix a crash when we can't connect to PG

The ActiveRecord railtie isn't really necessary as far as we can tell. This commit fixes all our currently known no-database woes by the simple expedient of rm'ing it.

#### Verification
 - [x] have a database.yml, turn off postgres
   - [x] start msfconsole
   - [x] verify no explosion
   - [x] verify warning that we can't connect
 - [x] have no database.yml
   - [x] start msfconsole
   - [x] verify no explosion
 - [x] have a database.yml with bad user/pass
   - [x] start msfconsole
   - [x] verify no explosion
   - [x] verify warning that database support will be disabled
 - [x] have accurate database.yml
   - [x] start msfconsole
   - [x] verify no explosion