BuildManager: Remove busy loop

This code froze the entire UI for the time that a build step took to
cancel, for no apparent reason.

Change-Id: I40f20b9875d92e8919debaeac2021d389b6b6c28
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/buildmanager.cpp

@@ -193,8 +193,6 @@ void BuildManager::cancel()
             return;
         d->m_canceling = true;
         d->m_currentBuildStep->cancel();
-        while (d->m_canceling)
-            QApplication::processEvents(QEventLoop::ExcludeUserInputEvents); // TODO: Is this really necessary?
     }
 }