[Seq] Remove `SkolemCache::mkSkolemSeqNth()` (#8643)

[Seq] Remove `SkolemCache::mkSkolemSeqNth()` (#8643)

This removes the `SkolemCache::mkSkolemSeqNth()` method. Instead of
handling the out-of-bounds case with a UF, we just use `seq.nth` in its
original form for the out-of-bounds case (and rely on the fact that the
equality engine is configured to perform congruence closure on that
kind). This is part of the changes for the paper on sequences.