leveldb: Deleted unused LevelDB::custom_block_cache_

In 960b992 LevelDB::custom_block_cache_ stopped being used, but
was not deleted from the header file.

Bug: None
Change-Id: I173ba7b9bcf12b4ec29ab7231c2864e1426cdfad
Reviewed-on: https://chromium-review.googlesource.com/699054
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506254}

components/leveldb_proto/leveldb_database.h

@@ -59,9 +59,8 @@ class LevelDB {
   DFAKE_MUTEX(thread_checker_);
 
   // The declaration order of these members matters: |db_| depends on |env_| and
-  // |custom_block_cache_| and therefore has to be destructed first.
+  // therefore has to be destructed first.
   std::unique_ptr<leveldb::Env> env_;
-  std::unique_ptr<leveldb::Cache> custom_block_cache_;
   std::unique_ptr<leveldb::DB> db_;
   base::HistogramBase* open_histogram_;