Remove limited implementation of __setitem__ from Array

[tomwhite]

I originally added a very limited form of __setitem__ that only accepts single element updates to support the Array API tests, that needed it at the time. Now this is no longer a requirement (https://hypothesis.readthedocs.io/en/latest/changes.html#v6-75-8), I propose removing the code since it isn't generally useful.

With this change we'd be able to treat arrays as immutable, which opens up more optimizations, such as making full a "virtual" (non-materialized) array (see #290, #247).

Also, if we decide we need to make arrays mutable in the future, we can do that in a more intentional way - perhaps by marking any mutated arrays as needing to be materialized to Zarr, so they can then be mutated.