Changed to only apply to F# targets, and to remove the old guid rather t...

...han replace.
For reference, until I add comments to the code.

*   We first check this is an F# project by checking the project imports to see if once of then ends with FSharp.Targets
*   Next we extract the ProjectTypeGuids and filter out the depreciated guid `4925A630-B079-445D-BCD4-3A9C94FE9307`
*   Now we rewrite the remaining guids back

**Note**: Xamarin Studio automatically adds the correct F# guid back in, as defined in the addin.xml file