Revert "Add initial firefox xpi prompt bypass." change to XPI module for Firefox 22-27

The versions of firefox that this bypass worked for are now covered by the exploit [Firefox WebIDL Privileged Javascript Injection](http://www.rapid7.com/db/modules/exploit/multi/browser/firefox_webidl_injection), so you might as well get a shell instead. Otherwise this code is bloating the module and adding a ~600ms delay to install, so I am removing it.

#### Verification
- [x] `msf> use exploits/multi/browser/firefox_xpi_bootstrapped_addon`
- [x] `msf> run`
- [x] Get a shell on any firefox version