pw_kvs: Remove unused working buffer

Removed the KVS working buffer that is not being used. It looks like
this is not needed anymore.

Change-Id: Iec4e87353fa13b1f7fee2a1497187d48b2252215
diff --git a/pw_kvs/public/pw_kvs/key_value_store.h b/pw_kvs/public/pw_kvs/key_value_store.h
index 5122140..1e23c45 100644
--- a/pw_kvs/public/pw_kvs/key_value_store.h
+++ b/pw_kvs/public/pw_kvs/key_value_store.h
@@ -75,9 +75,6 @@
 
 class KeyValueStore {
  public:
-  // TODO: Rework entry relocation to not need a large buffer.
-  static constexpr size_t kWorkingBufferSizeBytes = (4 * 1024);
-
   // KeyValueStores are declared as instances of
   // KeyValueStoreBuffer<MAX_ENTRIES, MAX_SECTORS>, which allocates buffers for
   // tracking entries and flash sectors.