pebble: remove specialized getIter implementation

`getIter` was a specialized `internalIterator` that lazily walked LSM
levels (batch → memtables → L0 sublevels → L1..L6) for a single `Get`,
wrapped in a thin `Iterator` shell purely for MERGE / range-deletion
handling. CockroachDB does not use `DB.Get` on hot paths, so the
specialization's maintenance cost outweighed its benefit.

Replace it with a thin `getInternal` that builds a regular `*Iterator`
via `DB.newIter`, calls `SeekPrefixGE(key)`, checks
`Comparer.Equal(iter.Key(), key)`, and returns the iterator itself as
the `io.Closer`.