Revert "Use dedicated task executor"

This reverts commit 3ed469d863.

app/src/main/java/eu/faircode/email/SimpleTask.java

@@ -70,7 +70,6 @@ public abstract class SimpleTask<T> implements LifecycleObserver {
     @SuppressLint("StaticFieldLeak")
     private static Context themedContext = null;
     private static final List<SimpleTask> tasks = new ArrayList<>();
-    private static final ExecutorService globalExecutor = Helper.getBackgroundExecutor(0, "tasks");
 
     private static final int REPORT_AFTER = 15 * 60 * 1000; // milliseconds
 
@@ -125,7 +124,10 @@ public abstract class SimpleTask<T> implements LifecycleObserver {
             wl = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, BuildConfig.APPLICATION_ID + ":task");
         }
 
-        return (localExecutor == null ? globalExecutor : localExecutor);
+        if (localExecutor != null)
+            return localExecutor;
+
+        return Helper.getParallelExecutor();
     }
 
     @NonNull