rtlwifi: remove CONNECTION_MONITOR flag

To use keep-alive mechanism in mac80211 stack, since driver supports
reporting accurate nullfunc frame tx ack now.

Signed-off-by: Tzu-En Huang <tehuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>