leveldb: Deleted leveldb_env::SharedReadCache.

In 328d3bee use of leveldb_env::SharedReadCache ceased, but it was
never removed from env_chromium.h.

Bug: None
Change-Id: Idf0e5b16a84b8d3747f70026ffaa55d2cf57dc5b
Reviewed-on: https://chromium-review.googlesource.com/696037Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Chris Mumford <cmumford@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505911}

third_party/leveldatabase/env_chromium.h

@@ -80,14 +80,6 @@ struct Options : public leveldb::Options {
   Options();
 };
 
-enum class SharedReadCache {
-  // Use for databases whose access pattern is dictated by browser code.
-  Default,
-  // Use for databases whose access pattern is directly influenced by Web APIs,
-  // like Indexed DB, etc.
-  Web,
-};
-
 const char* MethodIDToString(MethodID method);
 
 leveldb::Status MakeIOError(leveldb::Slice filename,