removed old chat message list custom accessibility functionality

bigbluebutton-client/locale/en_US/bbbResources.properties

...client/src/org/bigbluebutton/main/events/ShortcutEvent.as

.../src/org/bigbluebutton/main/views/ShortcutHelpWindow.mxml

...t/src/org/bigbluebutton/modules/chat/model/ChatMessage.as

...ent/src/org/bigbluebutton/modules/chat/views/ChatBox.mxml