MAINTAINERS: remove outdated entry for crypto/rng.c

Lore shows no emails from Neil on linux-crypto since 2020. Without the
listed person being active, this MAINTAINERS entry provides no value,
and actually is a bit confusing because while it is called the
"CRYPTOGRAPHIC RANDOM NUMBER GENERATOR", it is not the CRNG that is
normally used (drivers/char/random.c) which has a separate entry.
Remove this entry, so crypto/rng.c will just be covered by "CRYPTO API".

Cc: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>