back to SSE

Given the current level of optimization and that AVX will be not used in production in 2015 we revert to the SSE version for all vectors.
(also removed an obsolete "pretended to be" SSE-specific code)

Purely technical, no regression expected.

Speed up expected and observed in few routines using double precision four vectors.
I'm fine with them being removed. We can always get them back via git if we ever need them.