Remove TODO comments with itemset and newbyteorder methods of dpnp.ndarray

[antonwolfy]

Currently itemset and newbyteorder methods are not implemented for dpnp.ndarray, but TODO comment was left in the code where it was expected to have the implementation.
Since NumPy 2.0 removed these methods from numpy.ndarray it looks as not needed to implement them in dpnp.

So the PR proposes to clean up the code and to remove comments per itemset and newbyteorder methods, since they are not going to be implemented for dpnp.ndarray anymore.
The link of the methods are also removed from generated dpnp documentation.

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