Revert "crypto/sha512: add WriteString and WriteByte method"

Revert "crypto/sha512: add WriteString and WriteByte method"

This reverts CL 483816

Reason for revert: can cause cgo errors when using boringcrypto.  See #59954.

For #38776
For #59954

Change-Id: I23a2a1f0aed2a08b73855b5038ccb24a4d0a02c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/492355
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
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