If I read right you want to uninstall FSX ? You should have the disc. Put it in your pc then it will give you the option to install , repair or uninstall. Click uninstall and it will do the work for you. If you want to get rid of everything then don't forget to go in Microsoft games folder and you should be left with an fsx folder which you can delete.
Go to Control panel / programs / uninstall a program and look for FlyInside Alpha. Uninstall it.
Then reinstall it making sure it goes to Steam/steamapps/common/fsx or where ever you have it. But IT WILL be under steam. This should uninstall it from both versions if you installed it twice.
Just make sure it points to the Steam FSX when you re install it mate. Hope this makes sense ?
If you have any trouble with the uninstaller, you can delete "d3d9.dll" from the FSX folder of your choice. That'll render FlyInside 100% disabled. Using the uninstaller is your best bet though, because it cleans up the other cruft FlyInside installs.