Fix unused variable warning in ieee80211_proto.c

With clang 15, the following -Werror warning is produced:

sys/net80211/ieee80211_proto.c:1070:34: error: variable 'num_mixed' set but not used [-Werror,-Wunused-but-set-variable]
        int num_vaps = 0, num_pure = 0, num_mixed = 0;
                                        ^

The 'num_mixed' variable was in ieee80211_proto.c when the function
vap_update_ht_protmode() was added, but it was never used for anything,
so remove it.

MFC after: 3 days

(cherry picked from commit 9319211f96c6f11959c76f1d565a744ab37b0031)