[macOS] Eliminate macOS 10.14 availability check (flutter#37490)

Now that the macOS embedders have all been updated to use a minimum
macOS SDK of 10.14, eliminate the remaining @available checks dependent
on that version.

Issue: flutter/flutter#114445

shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMac.mm

@@ -216,30 +216,6 @@
       break;
     }
     case ui::AXEventGenerator::Event::LIVE_REGION_CHANGED: {
-      if (@available(macOS 10.14, *)) {
-        // Do nothing on macOS >=10.14.
-      } else {
-        // Uses the announcement API to get around OS <= 10.13 VoiceOver bug
-        // where it stops announcing live regions after the first time focus
-        // leaves any content area.
-        // Unfortunately this produces an annoying boing sound with each live
-        // announcement, but the alternative is almost no live region support.
-        NSString* announcement = [[NSString alloc]
-            initWithUTF8String:mac_platform_node_delegate->GetLiveRegionText().c_str()];
-        NSDictionary* notification_info = @{
-          NSAccessibilityAnnouncementKey : announcement,
-          NSAccessibilityPriorityKey : @(NSAccessibilityPriorityLow)
-        };
-        // Triggers VoiceOver speech and show on Braille display, if available.
-        // The Braille will only appear for a few seconds, and then will be replaced
-        // with the previous announcement.
-        events.push_back({
-            .name = NSAccessibilityAnnouncementRequestedNotification,
-            .target = [NSApp mainWindow],
-            .user_info = notification_info,
-        });
-        break;
-      }
       // Uses native VoiceOver support for live regions.
       events.push_back({
           .name = kAccessibilityLiveRegionChangedNotification,