This does show a trace that you did get the achievement in the Steam Community blotter, though. This wouldn't be a problem if there weren't at least 3 other achievements that came before that, but thanks to those, it is a bit obvious that there is something fishy going on.
Now, whether Valve will bother enforcing this kind of rule is another question entirely, but going by the Cheater's Lament incident and all the fake idle servers, I'd remain wary.
EDIT: Speaking of traces, removing the game content through Steam still leaves the nProtect GameGuard service in all its rootkit-esque damnation. In your Steam directory, go to /steamapps/common, delete the ava directory, then under Control Panel -> Administrative Tools -> Services search for an nProtect entity; double-click it, stop the service if it isn't already, and set Startup Type to Disabled.
Afterwards search your hard drive for *.des files. Go to GameMon.des's directory (under /Windows/SysWOW64 on 64-bit Win7, unsure of other OSes); delete that, nppt9x.vxd, and npptNT2.sys.
To finalise things, go to regedit and go to HKEY_LOCAL_MACHINE\\SYSTEM\\CurrentControlSet\\Services\\ and delete the entire npggsvc branch. It'll take effect on reboot.