crypto: engine - remove {prepare,unprepare}_crypt_hardware callbacks

The {prepare,unprepare}_crypt_hardware callbacks were added back in 2016
by commit 735d37b5 ("crypto: engine - Introduce the block request
crypto engine framework"), but they were never implemented by any driver.
Remove them as they are unused.

Since the 'engine->idling' and 'was_busy' flags are no longer needed,
remove them as well.

Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>