Revert "Support SIMD or non-SIMD build"

This reverts commit d2e031bdeefc03807dd491dfea7378feafa113a2.
Clean the code to focus on SIMD path.
optimize the b2Vec2 constructor and Set with two floats.
optimize b2Vec.Length()
Minimize SIMD code, LLVM do good job for vectorization.
Bring back some SIMD code.