Remove a large number of unused structs and functions

Remove a large number of unused structs and functions

These have been found by Clion's Inspect Code functionality and have
been verified by hand.  A few functions like buf_read_u32 have been
kept since they still feel being useful while currently not being used.

Change-Id: I0d96ee06c355c6a5ce082af23921e329d3efae33
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20241018063717.14629-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg29594.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>