crypto: n2 - Remove unused struct 'n2_skcipher_request_context'

'n2_skcipher_request_context' was added in
commit 23a6564a ("crypto: niagara2 - switch to skcipher API")
but never used.
Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>