cut unnecessary ctor args to folly::ThreadLocal

Summary: `folly::ThreadLocal` accepts a constructor function to construct instances, but it is not necessary when it does the same thing as the default constructor function: make and return a new instance of the type as `[] { return new T(); }`.

Reviewed By: Gownta

Differential Revision: D55784505

fbshipit-source-id: 5239cf12a73894e323f084f9848ca541d7bba6a6

Author: yfeldblum

cachelib/cachebench/workload/OnlineGenerator.cpp

@@ -28,7 +28,6 @@ namespace cachebench {
 
 OnlineGenerator::OnlineGenerator(const StressorConfig& config)
     : config_{config},
-      key_([]() { return new std::string(); }),
       req_([&]() { return new Request(*key_, dummy_.begin(), dummy_.end()); }) {
   for (const auto& c : config_.poolDistributions) {
     if (c.keySizeRange.size() != c.keySizeRangeProbability.size() + 1) {