wifi: cfg80211: remove 5/10 MHz channel support

wifi: cfg80211: remove 5/10 MHz channel support

Remove WIPHY_FLAG_SUPPORTS_5_10_MHZ and 5/10 MHz channel
width support. We contemplated this back in early 2023
and didn't do it yet, but nobody stepped up to maintain
it.

It's already _mostly_ dead code since it can really only
be used for AP and maybe IBSS and monitor, but not on a
client since there's no way to scan (and hasn't been in
a very long time, if ever), so the only thing that ever
could really happen with it was run syzbot and trip over
assumptions in the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Lachlan Hodges <lachlan.hodges@morsemicro.com>
Link: https://patch.msgid.link/20260529084502.080c5885f0b7.I77cc94485b523c3c006005b9233db13cd4e077b3@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>