Disconnect the texinfo admin guide from the build

Disconnect the texinfo admin guide from the build

Its content has been migrated to or superseded by the reST documentation,
essentially entirely in krb_admins.

A few portions of the texinfo document are simply no longer relevant
and do not need to be migrated.  In particular:
Information about reporting bugs lives on k5wiki.kerberos.org.
General Kerberos concepts/introduction will be elsewhere in the tree.
We do not need to document the time zones accepted by kadmin.
We do not need a table of the various error codes and strings in our
formal documentation.
A complete description of the layout of our source tree is not useful
or relevant to most Kerberos administrators.

ticket: 7408