LinuxKPI: 802.11: reduce code duplication introducing lkpi_remove_chanctx()

We have the same code three times, so factor it out into its own
function to make it easier to maintain.

Sponsored by: The FreeBSD Foundation
PR: 280546
Tested by: Oleksandr Kryvulia (shuriku shurik.kiev.ua)
Tested by: Oleg Nauman (oleg.nauman gmail.com) [rtw88]
Differential Revision: https://reviews.freebsd.org/D49734

(cherry picked from commit 50d826be664ef35065e8c4c4966252f2dfde9cb9)