Remove the F# mime type definition as it is defined in the F# binding

Having the mime type defined twice means the one here is loaded first
and no icons are shown for F# files or projects.
Update the F# mime type to match those defined in the F# binding

text/x-fsharp-fs, text/x-fsharp-fsx, text/fsharp-fsi are the new F#
code mime types.