net: 802: Remove unused p8022 code

net: 802: Remove unused p8022 code

p8022.c defines two external functions, register_8022_client()
and unregister_8022_client(), the last use of which was removed in
2018 by
commit 7a2e838d28cf ("staging: ipx: delete it from the tree")

Remove the p8022.c file, it's corresponding header, and glue
surrounding it.  There was one place the header was included in vlan.c
but it didn't use the functions it declared.

There was a comment in net/802/Makefile about checking
against net/core/Makefile, but that's at least 20 years old and
there's no sign of net/core/Makefile mentioning it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Link: https://patch.msgid.link/20250418011519.145320-1-linux@treblig.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>