Switch SerialExecutor to default to the immutable global executor

Summary: Use the global immutable executor by default and remove the default entirely for the deprecated form.

Reviewed By: yfeldblum

Differential Revision: D26414638

fbshipit-source-id: b48fe4683165e7f29e8003560f2917e143138bb0

folly/executors/SerialExecutor.h

@@ -58,7 +58,7 @@ class SerialExecutor : public SequencedExecutor {
   SerialExecutor& operator=(SerialExecutor&&) = delete;
 
   static KeepAlive<SerialExecutor> create(
-      KeepAlive<Executor> parent = getKeepAliveToken(getCPUExecutor().get()));
+      KeepAlive<Executor> parent = getGlobalCPUExecutor());
 
   class Deleter {
    public:
@@ -76,7 +76,7 @@ class SerialExecutor : public SequencedExecutor {
 
   using UniquePtr = std::unique_ptr<SerialExecutor, Deleter>;
   [[deprecated("Replaced by create")]] static UniquePtr createUnique(
-      std::shared_ptr<Executor> parent = getCPUExecutor());
+      std::shared_ptr<Executor> parent);
 
   /**
    * Add one task for execution in the parent executor