ppp: mppe: Revert "ppp: mppe: Add softdep to arc4"

Commit 0e5a610b ("ppp: mppe: switch to RC4 library interface"),
which was merged through the crypto tree for v5.3, changed ppp_mppe.c to
use the new arc4_crypt() library function rather than access RC4 through
the dynamic crypto_skcipher API.

Meanwhile commit aad1dcc4 ("ppp: mppe: Add softdep to arc4") was
merged through the net tree and added a module soft-dependency on "arc4".

The latter commit no longer makes sense because the code now uses the
"libarc4" module rather than "arc4", and also due to the direct use of
arc4_crypt(), no module soft-dependency is required.

So revert the latter commit.

Cc: Takashi Iwai <tiwai@suse.de>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>