[SLP] delete unused pairwise reduction option

SLP tries to model 2 forms of vector reductions: pairwise and splitting.
From the cost model code comments, those are defined using an example as:

  /// Pairwise:
  ///  (v0, v1, v2, v3)
  ///  ((v0+v1), (v2+v3), undef, undef)
  /// Split:
  ///  (v0, v1, v2, v3)
  ///  ((v0+v2), (v1+v3), undef, undef)

I don't know the full history of this functionality, but it was partly
added back in D29402. There are apparently no users at this point (no
regression tests change). X86 might have managed to work-around the need
for this through cost model and codegen improvements.

Removing this code makes it easier to continue the work that was started
in D87416 / D88193. The alternative -- if there is some target that is
silently using this option -- is to move this logic into LoopUtils. We
have related/duplicate functionality there via llvm::createTargetReduction().

Differential Revision: https://reviews.llvm.org/D93860