staging: wfx: drop 'secure link' feature

staging: wfx: drop 'secure link' feature

The Secure Link (slk) feature allows to encrypt (and authenticate) the
traffic between the host and the device. The official implementation of
this feature relies on mbedTLS. For that reason, this implementation is
not included in the current driver. To be included, the implementation
has to rely on kernel crypto API instead of mbedTLS.

So, for now, the driver contained stub functions waiting for the new
implementation based on kernel crypto API.

This new implementation represent a bunch of work and it is unlikely to
be done in near future.  Moreover, we strongly dislike to maintain
useless code. So, drop all the code related to secure link.

Whoever wants to implement secure link should revert this patch before
starting to work on it.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200907101521.66082-6-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>