mac80211: remove vif debugfs driver callbacks

This basically reverts commit b207cdb0.

Now is possible to use drv_{add,remove}_interface() and vif->debugfs_dir
to create/remove per interface debugfs files. Remove redundant
callbacks.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>