Revert "Define the use of SHA-3 for SRTP authentication tags"

Phil doesn't want to define this yet since SHA-3 may be slower than
Skein and for auth tags this is important.  I mentioned that I'll
probably benchmark SHA-3 sometime in this configuration for
comparison.

This reverts commit 92f016586d0e1b63afb5dd7ddd47e170448e7dd6.