chore(ndb): remove redundant --fail-under=100 from cover session in noxfile

Author: ohmayr

packages/google-cloud-ndb/noxfile.py

@@ -185,7 +185,7 @@ def cover(session):
     test runs (not system test runs), and then erases coverage data.
     """
     session.install("coverage", "pytest-cov")
-    session.run("coverage", "report", "--show-missing", "--fail-under=100")
+    session.run("coverage", "report", "--show-missing")
 
     session.run("coverage", "erase")