Cut dead reference to executorLock_ in Futures test

Summary: [Folly] Cut dead reference to `executorLock_` in Futures test - this field was recently removed.

Reviewed By: LeeHowes

Differential Revision: D7725446

fbshipit-source-id: a522b66da77bbefeb69970bee20feea46dcc72a5

folly/futures/test/CoreTest.cpp

@@ -32,7 +32,6 @@ TEST(Core, size) {
     std::atomic<bool> active_;
     std::atomic<bool> interruptHandlerSet_;
     folly::MicroSpinLock interruptLock_;
-    folly::MicroSpinLock executorLock_;
     int8_t priority_;
     Executor* executor_;
     std::shared_ptr<RequestContext> context_;