[mlir][Vector] Remove lhs and rhs masks from vector.contract

[mlir][Vector] Remove lhs and rhs masks from vector.contract

This patch removes the historical lhs and rhs masks in vector.contract,
now that vector.mask supports vector.contract and the lhs and rhs masks
are barely supported by all the vector.contract lowerings and
transformations.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D144430