swarm: clean up unused private types and functions (#17989)

* swarm: clean up unused private types and functions

Those that were identified by code inspection tool.

* swarm/storage: move/add Proximity GoDoc from deleted private function

The mentioned proximity() private function was deleted in:
1ca8fc1e6f

swarm/storage/ldbstore.go

@@ -39,7 +39,6 @@ import (
 	"github.com/ethereum/go-ethereum/swarm/log"
 	"github.com/ethereum/go-ethereum/swarm/storage/mock"
 	"github.com/syndtr/goleveldb/leveldb"
-	"github.com/syndtr/goleveldb/leveldb/opt"
 )
 
 const (
@@ -72,13 +71,6 @@ var (
 	ErrDBClosed = errors.New("LDBStore closed")
 )
 
-type gcItem struct {
-	idx    *dpaDBIndex
-	value  uint64
-	idxKey []byte
-	po     uint8
-}
-
 type LDBStoreParams struct {
 	*StoreParams
 	Path string
@@ -961,15 +953,3 @@ func (s *LDBStore) SyncIterator(since uint64, until uint64, po uint8, f func(Add
 	}
 	return it.Error()
 }
-
-func databaseExists(path string) bool {
-	o := &opt.Options{
-		ErrorIfMissing: true,
-	}
-	tdb, err := leveldb.OpenFile(path, o)
-	if err != nil {
-		return false
-	}
-	defer tdb.Close()
-	return true
-}