crypto: arc4 - mark ecb(arc4) skcipher as obsolete

Cryptographic algorithms may have a lifespan that is significantly
shorter than Linux's, and so we need to start phasing out algorithms
that are known to be broken, and are no longer fit for general use.

RC4 (or arc4) is a good example here: there are a few areas where its
use is still somewhat acceptable, e.g., for interoperability with legacy
wifi hardware that can only use WEP or TKIP data encryption, but that
should not imply that, for instance, use of RC4 based EAP-TLS by the WPA
supplicant for negotiating TKIP keys is equally acceptable, or that RC4
should remain available as a general purpose cryptographic transform for
all in-kernel and user space clients.

Now that all in-kernel users that need to retain support have moved to
the arc4 library interface, and the known users of ecb(arc4) via the
socket API (iwd [0] and libell [1][2]) have been updated to switch to a
local implementation, we can take the next step, and mark the ecb(arc4)
skcipher a...