[CMake] Remove CGALPlugin from supported plugins

[bakpaul]

As CGaL hasn't been compiling without crash on ubuntu for a long time now, it is time to deactivate it.
To do this smoothly, I added a nightly here :
sofa-framework/CGALPlugin#42

And python example on how to replace this plugin by using the CGaL bindings directly here :
sofa-framework/SofaPython3#567
--> except for the generation from image, as the bindings don't exist for this part of CGaL.

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