Remove `disableNewTaskModel` failsafe.

lib/src/driver.dart

Index: lib/src/driver.dart
diff --git a/lib/src/driver.dart b/lib/src/driver.dart
index 23bc8269a271a9aa45002a7bc28cac1f5453e851..752cb1e87597f572754ca4169c4936a35a1a6f7d 100644
--- a/lib/src/driver.dart
+++ b/lib/src/driver.dart
@@ -231,9 +231,6 @@ class Driver {
     if (options.enableSuperMixins != _previousOptions.enableSuperMixins) {
       return false;
     }
-    if (options.disableNewTaskModel != _previousOptions.disableNewTaskModel) {
-      return false;
-    }
     return true;
   }
 
@@ -388,8 +385,6 @@ class Driver {
       return;
     }
     _previousOptions = options;
-    // Determine whether the new task model should be used.
-    AnalysisEngine.instance.useTaskModel = !options.disableNewTaskModel;
     // Choose a package resolution policy and a diet parsing policy based on
     // the command-line options.
     SourceFactory sourceFactory = _chooseUriResolutionPolicy(options);