The "delete marker" and "delete keyframe" hotkey are the same - both "x" or "delete". In effect that means, that "delete marker" has no hotkey. In some cases one key press results in Blender asking me if i want to delete a marker after i already deleted a keyframe.
To recreate change view to the animation screen and check the menus for key and marker and their hotkeys. It is that way in all windows with those menus (Graph Editor, Dope Sheet, Action Editor ...)
Blender 2.71
Windows 7, 64bit