Simplify logic in default qubit sampling

[albi3ro]

Context:

We added some bepoke grouping logic back when developing the sampling logic because split_non_commuting was not general enough for our purposes. Now we do have split_non_commuting and diagonalize_measurements, so we can remove the bepoke grouping logic.

Description of the Change:

Removes the unique grouping logic in favor of simply relying on split_non_commuting and diagonalize_measurements.

Benefits:

Simpler code, less duplication.

Possible Drawbacks:

Related GitHub Issues:

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