Fix Process disposal

Author: andrewlock

tracer/test/Datadog.Trace.TestHelpers/ProcessHelper.cs

@@ -130,8 +130,6 @@ public virtual void Dispose(int waitForExitTimeout)
                     // Ignore exceptions when killing the process, as it may have already exited
                 }
             }
-
-            Process.Dispose();
         }
     }
 }