[Unity plugin] Remove sensor noise properties

[Balint-H]

Remove references in MjGlobalOptions and sensor base class to the deprecated sensornoise attribute.

[Balint-H]

I think it could be easily misleading, e.g. they have learned some RL controller but want to make it more robust with sensor noise, they see the option and just put a value there without it actually having the intended effect. In any component they use to collect sensor readings with, they can inject noise there just as well.

[erez-tom]

Could you please run the tests? Both MjBaseSensorTests and MjcfImporterTests make references to this now-absent field.

[Balint-H]

I forgot about the test framework implementation in the package! From now on I'll make sure to run them before a PR, and I'll amend this one.