[MLIR] Simplify logic in `parseDynamicIndexList` (NFC)

We can use `parseCommaSeparatedList` to simplify the logic of
`parseDynamicIndexList`. We don't need to explicitly check delimiters
and comma anymore, this is done for us by `parseCommaSeparatedList`.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D138694