MAC80211: Remove unused MAC80211_VERBOSE_SPECT_MGMT_DEBUG.

Remove this unused Kconfig variable, which Intel apparently once
promised to make use of but never did.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>