crypto: jitterentropy - remove unnecessary information from a comment

The clocksource does not provide clocksource_register() function since
f8935983 commit (clocksource: Mostly kill clocksource_register()), so
let's remove unnecessary information about this function from a comment.

Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>