Remove hnbgw_vty_reference from the source tree.
In manuals/Makefile.am use the new BUILT_REFERENCE_XML feature recently added
to osmo-gsm-manuals, and add a build target to generate the XML using the new
osmo-hnbgw --vty-ref-xml cmdline switch.
Change-Id: Ia1e978ad9139c41368cd2720c339e1738d345c4a
Depends: I613d692328050a036d05b49a436ab495fc2087ba
Related: OS#5041