Remove error generating redundant statement from firstboot script

I've noticed that the firstboot script was causing an error during startup:
```
/root/firstboot.sh: 9: [: trusty: unexpected operator
```

The reason for it is that we're using /bin/sh as the shell, but using '==' as the string comparison, which isn't supported - this fails on both lucid as well as trusty. 

The consequence of the failure is that, regardless of the value in $DISTRIB_CODENAME, the else codepath is followed, and both lucid AND trusty support specifying "--exclude=dev" during ifup/ifdown statements.

Therefore, removing the redundant bits.