doc: Remove chapter on backporting MCUmgr fixes to 2.7

2.7 is EOL; the page is pointless.

Signed-off-by: Dominik Ermel <[email protected]>

Author: de-nordic

doc/services/device_mgmt/index.rst

@@ -9,7 +9,6 @@ Device Management
     mcumgr.rst
     mcumgr_handlers.rst
     mcumgr_callbacks.rst
-    mcumgr_backporting.rst
     smp_protocol.rst
     smp_transport.rst
     dfu.rst