CSM: Remove redundant "toRad" function

[gkjohnson]

Removes the toRad functions and replaces it with MathUtils.degToRad.

I'm looking into making some other changes to the CSM example coming up including:

• Using named imports to be consistent with the other examples.
• Adding support for orthographic cameras for convenience
• Adding fade between cascades

@vhawk Let me know what you think!

[StrandedKitty]

It will be great if you improve my example. At some point I tried to implement smooth interpolation between cascades, but I haven't published it because it made shader code even more ugly.

[mrdoob]

Thanks!