Remove SHA3 references

Skein was one of the finalists in the NIST competition to develop a
new hash function called SHA-3. Skein made it all the way through to
the end but was ultimately not selected. Since skeinsum uses the
reference implementation that was submitted to NIST for the SHA-3
competition it contained references to SHA3. This commit removes them,
since Skein wasn't selected and isn't SHA-3.