doc: Drop the docbook admin guide

It has been converted to Markdown.

Change-Id: Idc4b2de7e79aa955d1d3cac96a9ee653ccf9fbd9
Signed-off-by: Lubomir Rintel <lubo.rintel@gooddata.com>
Reviewed-on: http://review.gluster.org/5473
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>