rt2x00: remove confusing AGC register

Register 66 was causing issues on RT6352 if set to the same value as
in MTK driver. With 1c reg value device was working fine in both HT20
and HT40 modes.

Signed-off-by: Tomislav Požega <pozega.tomislav@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>