Fix notebooks.toml (Qiskit#1386)

We didn't notice this until we tried executing notebooks that submit
jobs.

scripts/nb-tester/notebooks.toml

@@ -53,6 +53,5 @@ notebooks_that_submit_jobs = [
 # memory on a reasonable device.
 notebooks_no_mock = [
     "docs/start/hello-world.ipynb",
-    "tutorials/combining-error-mitigation-estimator/combining-error-mitigation-estimator.ipynb",
     "tutorials/combine-error-mitigation-options-with-the-estimator-primitive/combine-error-mitigation-options-with-the-estimator-primitive.ipynb",
 ]