Remove unused low_pri_write_rate_limiter_ (facebook#6068)

Summary:
`low_pri_write_rate_limiter_` is not being used. Removing. `WriteController` has an internal low_pri rate limiter which is the real rate limiter for low-pri writes.
Pull Request resolved: facebook#6068

Test Plan: make

Differential Revision: D18664120

fbshipit-source-id: dfe3e4de033cf3522b67781b383aad7d0936034c

db/db_impl/db_impl.cc

@@ -141,8 +141,6 @@ void DumpSupportInfo(Logger* logger) {
   ROCKS_LOG_HEADER(logger, "Fast CRC32 supported: %s",
                    crc32c::IsFastCrc32Supported().c_str());
 }
-
-int64_t kDefaultLowPriThrottledRate = 2 * 1024 * 1024;
 }  // namespace
 
 DBImpl::DBImpl(const DBOptions& options, const std::string& dbname,
@@ -178,11 +176,6 @@ DBImpl::DBImpl(const DBOptions& options, const std::string& dbname,
       write_thread_(immutable_db_options_),
       nonmem_write_thread_(immutable_db_options_),
       write_controller_(mutable_db_options_.delayed_write_rate),
-      // Use delayed_write_rate as a base line to determine the initial
-      // low pri write rate limit. It may be adjusted later.
-      low_pri_write_rate_limiter_(NewGenericRateLimiter(std::min(
-          static_cast<int64_t>(mutable_db_options_.delayed_write_rate / 8),
-          kDefaultLowPriThrottledRate))),
       last_batch_group_size_(0),
       unscheduled_flushes_(0),
       unscheduled_compactions_(0),

db/db_impl/db_impl.h

@@ -1831,8 +1831,6 @@ class DBImpl : public DB {
 
   WriteController write_controller_;
 
-  std::unique_ptr<RateLimiter> low_pri_write_rate_limiter_;
-
   // Size of the last batch group. In slowdown mode, next write needs to
   // sleep if it uses up the quota.
   // Note: This is to protect memtable and compaction. If the batch only writes