Refactor: RuntimeScheduler: Stop defaulting ctor args redundantly (#43954)

Summary:
Pull Request resolved: #43954

Now, all the defaulting is in RuntimeScheduler.h.

Changelog: [Internal]

Reviewed By: cipolleschi

Differential Revision: D55547900

fbshipit-source-id: 53678c6a0c83a47e3b0c1a51e388daadfa6bff5e

Author: RSNara

packages/react-native/ReactCommon/react/renderer/runtimescheduler/RuntimeScheduler_Legacy.h

@@ -22,8 +22,7 @@ class RuntimeScheduler_Legacy final : public RuntimeSchedulerBase {
  public:
   explicit RuntimeScheduler_Legacy(
       RuntimeExecutor runtimeExecutor,
-      std::function<RuntimeSchedulerTimePoint()> now =
-          RuntimeSchedulerClock::now);
+      std::function<RuntimeSchedulerTimePoint()> now);
 
   /*
    * Not copyable.

packages/react-native/ReactCommon/react/renderer/runtimescheduler/RuntimeScheduler_Modern.h

@@ -23,8 +23,7 @@ class RuntimeScheduler_Modern final : public RuntimeSchedulerBase {
  public:
   explicit RuntimeScheduler_Modern(
       RuntimeExecutor runtimeExecutor,
-      std::function<RuntimeSchedulerTimePoint()> now =
-          RuntimeSchedulerClock::now);
+      std::function<RuntimeSchedulerTimePoint()> now);
 
   /*
    * Not copyable.