Witch is simply a System Settings pane with a background task that catches your kestrokes (in order to display the switcher window). To uninstall Witch, first disable it (remove the checkmark next to Enable Witch in Witch's Settings pane). This stops the background task. Next, return to the top-level of System Settings, control-click on Witch, and select Remove "Witch" Settings Pane from the contextual menu (it's the only entry).

Quit System Settings, and you're done - no need to logout/login or restart.

If you want to get rid of Witch's static files—its settings, etc.—delete these two items:

1. The Witch folder in your user's Library → Application Support folder.
2. The com.manytricks.Witch.plist file in your user's Library → Preferences folder.

Keep in mind that if you install Witch again in the future, you'll have to create all your settings again.