wil6210: remove SSID debugfs

This driver shouldn't be using wdev->ssid to start with, as
it's more or less an internal field in cfg80211 used for
various purposes. Reading it is possible through nl80211,
even if that's not really what we should be doing there
for anything but AP type interfaces.

It *really* shouldn't allow modifying it!

Remove the whole debugfs entry.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>