Hide "Rig" menu when using MHX

[Chuki Cicha]

When you convert a rig to MHX, the buttons under the Rig menu doesn’t show the proper bone layers when pressed. And also you already have the MHX Layers and MHX FK/IK Switch tabs. So Rig tab is useless when using MHX, so maybe it would be better to hide it when there’s a MHX rig selected.

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[Thomas Larsson]

Fixed in last commit