Reduce the number of SHA-512 C implementations from three to one.

Reduce the number of SHA-512 C implementations from three to one.

We currently have three C implementations for SHA-512 - a version that is
optimised for CPUs with minimal registers (specifically i386), a regular
implementation and a semi-unrolled implementation. Testing on a ~15 year
old i386 CPU, the fastest version is actually the semi-unrolled version
(not to mention that we still currently have an i586 assembly
implementation that is used on i386 instead...).

More decent architectures do not seem to care between the regular and
semi-unrolled version, presumably since they are effectively doing the
same thing in hardware during execution.

Remove all except the semi-unrolled version.

ok tb@