First, open a Terminal window.
Then, paste the following command, changing "[user]" to your user name on the Mac:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain system -domain [user]
If it executes with no errors, then relaunch the Finder. You should have a much cleaner "Open With >" menu.