keymap: Remove wlan from Dell

The Dell rfkill key is handled by hardware and the dell-laptop driver catches
the i8042 event in order to update the rfkill state. Sending wlan to userspace
will just result in userspace trying to revert the change the hardware has
just made.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Martin Pitt <martin.pitt@ubuntu.com>