Remove evidence of kitrap0d

This exploit has been causing crashes and BSODs on various systems and
hence is deemed too unstable to be included in the default deployment of
Meterpreter. `getsystem` should only contain code which attempts to get
SYSTEM privileges via safe means; it should not have exploits in it.

This commit removes kitrap0d from `getsystem`. The code has been moved to a
[windows local exploit in MSF](https://github.com/rapid7/metasploit-framework/pull/2633) instead.

Some other code fixes/tidies added as well.