Remove old or redundant SHA2 files

We had three sha2.h headers in different places.
The FreeBSD version, the Linux version and the generic solaris version.

The only assembly used for acceleration was some old x86-64 openssl
implementation for sha256 within the icp module.

For FreeBSD the whole SHA2 files of FreeBSD were copied into OpenZFS,
these files got removed also.

Tested-by: Rich Ercolani <rincebrain@gmail.com>
Tested-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #13741