crypto: skcipher - Remove SKCIPHER_REQUEST_ON_STACK()

Now that all the users of the VLA-generating SKCIPHER_REQUEST_ON_STACK()
macro have been moved to SYNC_SKCIPHER_REQUEST_ON_STACK(), we can remove
the former.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>