Remove unused seedsRefs from DuplicateListMerger.cc

`RecoTracker/FinalTrackSelectors/plugins/DuplicateListMerger.cc:196:49`
was reported by UBSan 25 times during full matrix.
```
DuplicateListMerger.cc:196:49: runtime error: variable length array
bound evaluates to non-positive value 0
```
Example workflows:
- 33.0 / step3
- 1319.0 / step3
- 1001.0 / step2
- 1002.0 / step2

`seedsRefs` is not used and removing it does not trigger any side effects.
Otherwise this is a potential spot for out-of-bounds writes to stack if
a possible code path if found.

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>