[CRYPTO] api: Remove unused functions

This patch removes the following no longer used functions:
- api.c: crypto_alg_available()
- digest.c: crypto_digest_init()
- digest.c: crypto_digest_update()
- digest.c: crypto_digest_final()
- digest.c: crypto_digest_digest()

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>