Remove dubious comment (#150608)

Follow-up to
https://github.com/flutter/flutter/pull/150349#discussion_r1648050036

I don't see how the code is preparing a union of anything here. I think
the comment is just broken.

This comment seems to have appeared in
https://github.com/flutter/flutter/pull/6093
It might have been mistakenly copied from
[here](a0227cab15 (diff-a6e4bf25b45ecb000db25d7343a44f018ff5302d4695eaa1277d804d2252c1f5R148))
(where it makes a lot more sense).

packages/flutter_tools/lib/src/commands/analyze_base.dart

@@ -208,7 +208,6 @@ class PackageDependencyTracker {
       }
     }
 
-    // prepare a union of all the .packages files
     if (dependencies.hasConflicts) {
       final StringBuffer message = StringBuffer();
       message.writeln(dependencies.generateConflictReport());