Drop job step and Create\Update alert handlers (#636)

* Add Delete Job Step implementation

* Update create\update agent alert handlers to use execution handler

* Cleanup create/update/delete operator request handlers

src/Microsoft.SqlTools.ServiceLayer/Agent/Jobs/JobHistoryItem.cs

@@ -64,7 +64,6 @@ namespace Microsoft.SqlTools.ServiceLayer.Agent
         private ILogEntry m_currentEntry = null;
         private int m_index = 0;
         private bool m_isClosed = false;
-        private TypedColumnCollection columnTypes;
         private IServiceProvider serviceProvider = null;
 
         private static string historyTableDeclaration   = "declare @tmp_sp_help_jobhistory table";