[iOS] Tweak note about OpenGL support on mac in a user facing log. (#54690)

We don't have an OpenGL backend on iOS.

Author: chinmaygarde

shell/platform/darwin/ios/ios_context_software.mm

@@ -38,7 +38,7 @@
   // message.
   NSLog(@"Flutter: Attempted to composite external texture sources using the software backend. "
         @"This backend is only used on simulators. This feature is only available on actual "
-        @"devices where OpenGL or Metal is used for rendering.");
+        @"devices where Metal is used for rendering.");
 
   // Not supported in this backend.
   return nullptr;