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
 - [ ] have a database.yml, turn off postgres
   - [ ] start msfconsole
   - [ ] verify no explosion
   - [ ] verify warning that we can't connect
 - [ ] have no database.yml
   - [ ] start msfconsole
   - [ ] verify no explosion
   - [ ] verify warning that database support will be disabled
 - [ ] have a database.yml with bad user/pass
   - [ ] start msfconsole
   - [ ] verify no explosion
   - [ ] verify warning that database support will be disabled
 - [ ] have accurate database.yml
   - [ ] start msfconsole
   - [ ] verify no explosion