mac80211: minstrel_ht: remove unused n_supported variable

mac80211: minstrel_ht: remove unused n_supported variable

clang with W=1 reports
net/mac80211/rc80211_minstrel_ht.c:1711:6: error: variable
  'n_supported' set but not used [-Werror,-Wunused-but-set-variable]
        int n_supported = 0;
            ^
This variable is not used so remove it.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230325132610.1334820-1-trix@redhat.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>