Remove dubious comment (#150608)

Follow-up to
flutter/flutter#150349 (comment)

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
flutter/flutter#6093
It might have been mistakenly copied from
[here](flutter/flutter@a0227ca#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());