libertas: remove unused Automatic Frequency Control command

It hasn't been hooked up to anything in a long time and it's not
even listed in any of the firmware documentation I have (and I
have v5.1, v8, v9, and v10).

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>