[mlir][vector] Remove default value in populateVectorMultiReductionLoweringPatterns

Having a default value for the lowering strategy of the multi-reduction op has proven
to be unexpected by users. This patch is dropping the default value so that users have
to explicitly choose the lowering strategy to be applied.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D115805