Stop enabling and disabling new_opmath

[lillian542]

Context:

In test_measurements_results.py, we explicitly enable and then disable new_opmath inside each test. Enabling new_opmath is no longer necessary, because it's the default. Disabling it at the end of each test means that when the test is completed, we are set to use legacy opmath.

Locally we run each test file separately, but in CI, any tests on the same runner that are run after this file are run with new_opmath disabled. This can lead to discrepancies between local and CI testing.

Description of the Change: We remove the explicit enabling and disabling of new_opmath

Benefits: We will be testing everything with new opmath in CI as intended, and local and CI tests will correspond better to each other.

Possible Drawbacks: This came up in a couple of PRs because added functionality failed with legacy opmath (otherwise this would never have been visible). Without this, we wouldn't have caught the legacy opmath incompatibility. But if we want to check for incompatibility with legacy opmath, we should add a workflow to the test matrix checking the whole thing regularly.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.98%. Comparing base (8c191be) to head (764059b). Report is 200 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #845 +/- ## ========================================== - Coverage 98.01% 97.98% -0.03% ========================================== Files 71 71 Lines 10429 10429 Branches 936 936 ========================================== - Hits 10222 10219 -3 - Misses 166 168 +2 - Partials 41 42 +1 ```

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[github-actions[bot]]

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