[CBM] Remove old documentation around the archive layout

What's the issue?
We have some old (and out-of-date depending on the version) documentation around the 'cbbackupmgr' archive layout. We shouldn't document the layout since it's subject to change (and do so regularly); it's also not something users should need to worry about, and definitely shouldn't be modifying (as documented in the README for newly created archives - Post CC).

What's the fix?
We should remove the 'cbbackupmgr-archivelayout.adoc' file and any relevant references.