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/pss/client/client_test.go

@@ -288,10 +288,6 @@ type testStore struct {
 	values map[string][]byte
 }
 
-func newTestStore() *testStore {
-	return &testStore{values: make(map[string][]byte)}
-}
-
 func (t *testStore) Load(key string) ([]byte, error) {
 	return nil, nil
 }