mac80211: remove unnecessary null test before debugfs_remove()

The debugfs_remove() function can safely take NULL parameters
so the additionally null test isn't required, and there's no
other reason to have it here, so remove it.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
[rewrite commit message, re-introduce blank line after assert]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>