`any_invocable`: Delete obsolete reference to proposed standard type

The comment calls if `any_invocable`, but the standard ultimately chose [`move_only_function`](https://en.cppreference.com/w/cpp/utility/functional/move_only_function/move_only_function).
And `absl::AnyInvocable` does not officially track the standard.

PiperOrigin-RevId: 631612663
Change-Id: I3f21f33cd6e2c6ce06ac92a8d2a68db6f942ea1b

absl/functional/internal/any_invocable.h

@@ -19,11 +19,11 @@
 
 ////////////////////////////////////////////////////////////////////////////////
 //                                                                            //
-// This implementation of the proposed `any_invocable` uses an approach that  //
-// chooses between local storage and remote storage for the contained target  //
-// object based on the target object's size, alignment requirements, and      //
-// whether or not it has a nothrow move constructor. Additional optimizations //
-// are performed when the object is a trivially copyable type [basic.types].  //
+// This implementation chooses between local storage and remote storage for   //
+// the contained target object based on the target object's size, alignment   //
+// requirements, and whether or not it has a nothrow move constructor.        //
+// Additional optimizations are performed when the object is a trivially      //
+// copyable type [basic.types].                                               //
 //                                                                            //
 // There are three datamembers per `AnyInvocable` instance                    //
 //                                                                            //