doc: remove obsolete performance comment

doc: remove obsolete performance comment

sha512sum can be faster than sha256sum.
E.g., ‘dd if=/dev/zero bs=1024k count=1024 | time sha256sum’
reports 8.16 user CPU seconds on my host, whereas sha512sum
consumes 5.45 seconds (Fedora x86-64 on an AMD Phenom II X4 910e).
Although sha512sum is still considerably slower on x86, a good
chunk of uses are on 64-bit hosts and anyway there’s little point
to scaring people away from sha512sum nowadays.
* doc/coreutils.texi (sha2 utilities): Remove obsolete comment.