Remove unused initializer from abstract DefaultKeepAliveExecutor

Summary: This constructor would never be called since the class is abstract (the method `add` from `Executor` is not implemented). This causes a warning which fails builds because of `-Werror`.

Reviewed By: yfeldblum

Differential Revision: D19345492

fbshipit-source-id: 20f185e0903a03acbde2e55e5a575e20deb5ad2a

folly/DefaultKeepAliveExecutor.h

@@ -29,8 +29,6 @@ namespace folly {
 /// threadsafe.
 class DefaultKeepAliveExecutor : public virtual Executor {
  public:
-  DefaultKeepAliveExecutor() : Executor() {}
-
   virtual ~DefaultKeepAliveExecutor() {
     DCHECK(!keepAlive_);
   }