ModifyFeature control: Allows use delkey on IE8

See the conversation on the DEV list: [OL 2.13 Modify Feature Example IE8/IE7 : Can't remove vertex](http://osgeo-org.1560.x6.nabble.com/OL-2-13-Modify-Feature-Example-IE8-IE7-Can-t-remove-vertex-td5072314.html)