[mlir][VectorType] Remove `numScalableDims` from the vector type

This is a follow-up of https://reviews.llvm.org/D153372 in which
`numScalableDims` (single integer) was effectively replaced with
`isScalableDim` bitmask.

This change is a part of a larger effort to enable scalable
vectorisation in Linalg. See this RFC for more context:
  * https://discourse.llvm.org/t/rfc-scalable-vectorisation-in-linalg/

Differential Revision: https://reviews.llvm.org/D153412