Revert "Runtime approach"

This reverts commit 7db9bdd.

Author: ncooke3

FirebaseCore/Extension/FIRHeartbeatLogger.h

@@ -40,7 +40,8 @@ typedef NS_ENUM(NSInteger, FIRDailyHeartbeatCode) {
 
 #ifndef FIREBASE_BUILD_CMAKE
 /// Returns the header value for the heartbeat logger via the given completion handler..
-- (void)asyncHeaderValueWithCompletionHandler:(void (^)(NSString *_Nullable))completionHandler;
+- (void)asyncHeaderValueWithCompletionHandler:(void (^)(NSString *_Nullable))completionHandler
+    API_AVAILABLE(ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0));
 
 /// Return the header value for the heartbeat logger.
 - (NSString *_Nullable)headerValue;
@@ -86,7 +87,8 @@ NSString *_Nullable FIRHeaderValueFromHeartbeatsPayload(FIRHeartbeatsPayload *he
 /// @note This API is thread-safe.
 /// @param completionHandler A completion handler to process the flushed payload of heartbeats.
 - (void)flushHeartbeatsIntoPayloadWithCompletionHandler:
-    (void (^)(FIRHeartbeatsPayload *))completionHandler;
+    (void (^)(FIRHeartbeatsPayload *))completionHandler
+    API_AVAILABLE(ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0));
 #endif  // FIREBASE_BUILD_CMAKE
 
 /// Gets today's corresponding heartbeat code.

FirebaseCore/Sources/FIRHeartbeatLogger.m

@@ -74,15 +74,11 @@ - (NSString *_Nullable)headerValue {
   return FIRHeaderValueFromHeartbeatsPayload([self flushHeartbeatsIntoPayload]);
 }
 
-- (void)asyncHeaderValueWithCompletionHandler:(void (^)(NSString *_Nullable))completionHandler {
-  if (@available(iOS 13.0, macOS 10.15, macCatalyst 13.0, tvOS 13.0, watchOS 6.0, *)) {
-    [self flushHeartbeatsIntoPayloadWithCompletionHandler:^(FIRHeartbeatsPayload *payload) {
-      completionHandler(FIRHeaderValueFromHeartbeatsPayload(payload));
-    }];
-  } else {
-    // no-op
-    completionHandler(nil);
-  }
+- (void)asyncHeaderValueWithCompletionHandler:(void (^)(NSString *_Nullable))completionHandler
+    API_AVAILABLE(ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0)) {
+  [self flushHeartbeatsIntoPayloadWithCompletionHandler:^(FIRHeartbeatsPayload *payload) {
+    completionHandler(FIRHeaderValueFromHeartbeatsPayload(payload));
+  }];
 }
 
 - (FIRHeartbeatsPayload *)flushHeartbeatsIntoPayload {
@@ -91,15 +87,11 @@ - (FIRHeartbeatsPayload *)flushHeartbeatsIntoPayload {
 }
 
 - (void)flushHeartbeatsIntoPayloadWithCompletionHandler:
-    (void (^)(FIRHeartbeatsPayload *))completionHandler {
-  if (@available(iOS 13.0, macOS 10.15, macCatalyst 13.0, tvOS 13.0, watchOS 6.0, *)) {
-    [_heartbeatController flushAsyncWithCompletionHandler:^(FIRHeartbeatsPayload *payload) {
-      completionHandler(payload);
-    }];
-  } else {
-    // no-op
-    completionHandler(nil);
-  }
+    (void (^)(FIRHeartbeatsPayload *))completionHandler
+    API_AVAILABLE(ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0)) {
+  [_heartbeatController flushAsyncWithCompletionHandler:^(FIRHeartbeatsPayload *payload) {
+    completionHandler(payload);
+  }];
 }
 #endif  // FIREBASE_BUILD_CMAKE