Getting Safari to forget Keywurl
From Keywurl's ReadMe:

Installation
Copy Keywurl.bundle into Library/Application Support/SIMBL/Plugins
under your home folder, or in /Library/Application
Support/SIMBL/Plugins in the hard drive root if you want Keywurl to be
enabled for all users on the computer.
Uninstalling
To uninstall, just delete the file you copied.

You said you mentioned that it was only installed locally, not globally, but I've had issues with other SIMBL plugins where I thought they were installed locally only to find another copy lurking at the system level.
So I'd check /Library/Application Support/SIMBL/Plugins as well, just to be sure. Note that this is a different hierarchy than the one mention in Gio's answer.
Make sure that you quit/relaunch Safari after you do this. A logout/restart might also help.