Remove most of the task model.

I run presubmit, and it does not look to me that any failin tests
are caused by this CL.
https://test.corp.google.com/ui#id=OCL:267824456:BASE:267824473:1567923135217:2380181a

R=brianwilkerson@google.com

Change-Id: I838f2174460e0d06c85bbb83de9037113c0b6393
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116252
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/instrumentation/instrumentation.dart

@@ -5,8 +5,6 @@
 import 'dart:async';
 import 'dart:convert';
 
-import 'package:analyzer/src/task/api/model.dart';
-
 /**
  * A container with analysis performance constants.
  */
@@ -122,17 +120,6 @@ class InstrumentationService {
    */
   String get _timestamp => new DateTime.now().millisecondsSinceEpoch.toString();
 
-  /**
-   * Log that the given analysis [task] is being performed in the given
-   * [context].
-   */
-  void logAnalysisTask(String context, AnalysisTask task) {
-    if (_instrumentationServer != null) {
-      _instrumentationServer
-          .log(_join([TAG_ANALYSIS_TASK, context, task.description]));
-    }
-  }
-
   /**
    * Log the fact that an error, described by the given [message], has occurred.
    */