plugin: Remove TODO from E2E test covering rotated -> not rotated

...when no new credentials are supplied. This was initially not covered
when the E2E tests were written but was added later once we the mocks
necessary to do unit testing to get error coverage.

plugin/e2e_test.go

@@ -358,11 +358,9 @@ func testPluginOnUpdateCatalog(
 
 	case newAccessKeyId == "" && rotated && !rotate:
 		// No new credentials have been provided, and we have previously
-		// rotated the credentials. This is actually an error.
-		//
-		// TODO: validate this scenario through unit testing. For now if
-		// we for some reason try to test this with this function, return
-		// an error.
+		// rotated the credentials. This is actually an error, but we
+		// don't test it here; it's covered in unit testing (see
+		// plugin_test.go).
 		require.FailNow("testing rotated-to-not-rotated scenario not implemented by this helper")
 
 	case newAccessKeyId == "" && !rotated && rotate: