misc: Remove lunarg_debug_marker extension

This device extensions was not completely supported in any layer. It had partial
support (intercept) in draw_state  and device_limits. This extension is being
changed and has a formal proposal in Khronos as debug_marker_ext. Removing
the existing extension since it is deprecated; once the debug_marker_ext gets
registered by Khronos, layers might want to start using the ObjectTag/ObjectName
commands to supplement error report messages in debug_report_ext. Since
debug_report_ext might get changed as part of Khronos review, don't add it yet.