Revert 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.

Forward port of #6573 in CMSSW_7_4_X. Merging directly, given the latter is already fully signed in CMSSW_7_3_X.