wifi: cfg80211/mac80211: remove wrong scan request n_channels

wifi: cfg80211/mac80211: remove wrong scan request n_channels

This (partially) reverts commits
 - 838c7b8f1f27 ("wifi: nl80211: Avoid address calculations via out of bounds array indexing")
 - f1d3334d604c ("wifi: cfg80211: sme: init n_channels before channels[] access")
 - 82bbe02b2500 ("wifi: mac80211: Set n_channels after allocating struct cfg80211_scan_request")

These commits all set the structure to be in an inconsistent
state, setting n_channels to some value before them actually
being filled in. That's fine for what the code does now, but
with the removal of __counted_by() in 444020f4bf06 ("wifi:
cfg80211: remove scan request n_channels counted_by") it's no
longer needed and it does leave a bit of a landmine there
since breaking out of some code to send the scan or something
would leave it wrong.

Remove the now superfluous n_channels settings.

Link: https://patch.msgid.link/20250718103237.59510b2384c5.Ied5ba9c5c49efc008f4491c8ca7a45858a83f064@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>