Revert "[SLP] Check with target before vectorizing GEP Indices."

This reverts commit 1387a13e.

This introduced performance regressions on AArch64, when the cost of a
vector GEP + extracts is offset by the benefits of vectorizing the rest
of the tree.

The test in llvm/test/Transforms/SLPVectorizer/AArch64/vector-getelementptr.ll
illustrates the issue. It was extracted from code that regressed a SPEC
benchmark by 15%.