[mlir][vector] Remove hooks deprecated pre Release/21 branch (#157806)
As mentioned on Discourse,
* https://discourse.llvm.org/t/psa-vector-standardise-operand-naming
I am removing the deprecated Vector hooks following the creation of the
release/21 branch.
The release/21 branch was created on July 15 (about two months ago) as
noted in the LLVM 21.x release announcement
* https://discourse.llvm.org/t/llvm-21-x-release-information-and-branching