[SLP][NFC]Remove handling of duplicates from getGatherCost
Duplicates are handled in BoUpSLP::processBuildVector (see TryPackScalars), support for duplicates in getGatherCost is not needed anymore.
Reviewers: hiraditya, RKSimon
Reviewed By: hiraditya, RKSimon
Pull Request: https://github.com/llvm/llvm-project/pull/135834
