odhcpd: remove the "filter_class" option

odhcpd: remove the "filter_class" option

The "filter_class" option was first introduced as part of the effort to support
the Homenet Control Protocol (HNCP, RFC7368, RFC7788). At first it was
hardcoded to filter out the user class "HOMENET", and later it was made
configurable, but there appears to be no other use-cases than Homenet. There's
also legacy comments in the odhcpd code pointing at filtering out homenet user
classes (the comments are misleading since "filter_class" is not set by
default).

Homenet is effectively dead. The "hnetd" daemon last saw any code changes in
2018, doesn't compile with current versions of GCC, and was removed from
OpenWrt altogether in 2025 [1].

The OpenWrt wiki also notes [2] that HNCP is effectively dead, and that the
upstream website [3] is gone.

Searching for the Homenet Control Protocol mostly returns results pointing
either:
	a) at the standards
	b) at the OpenWrt/hnetd project
	c) at various posts stating that it is dead

The IETF working group for Homenet is no more, and Éric Vyncke (Area Director
for the Internet Area of the IETF) mentioned wanting to avoid Homenet pitfalls
[4] (page 11) in the future of IPv6 autoconfig.

The current implementation is also contrary to the spirit of the RFCs
introducing the option (RFC3004 - DHCPv4; RFC8415, §21.15 - DHCPv6) where it is
described (example from RFC8415) as follows:

   The information contained in the data area of this option is
   contained in one or more opaque fields that represent the user class
   or classes of which the client is a member.  A server selects
   configuration information for the client based on the classes
   identified in this option.  For example, the User Class option can be
   used to configure all clients of people in the accounting department
   with a different printer than clients of people in the marketing
   department.

I.e., it's meant to be used in a manner similar to "tags" in dnsmasq.

Finally, the option has been undocumented for the whole existance of odhcpd.

So, remove this option.

[1] https://github.com/openwrt/routing/commit/85b868b3413a29da0bd6ecd3518c2d34a6ffb788
[2] https://openwrt.org/docs/guide-user/network/zeroconfig/hncp_configuration
[3] https://web.archive.org/web/20180831161552/http://homewrt.org/start
[4] https://ripe87.ripe.net/wp-content/uploads/presentations/102-20231130-RIPE-87-IPv6-from-IETF.pdf

Signed-off-by: David Härdeman <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/294
Signed-off-by: Álvaro Fernández Rojas <[email protected]>