deps [nfc]: Cut misplaced comment about gen-l10n

This isn't a helpful place for this information, because there's
no reason one would be looking in the pubspec at a time when one
needs it.  We have the same information about `flutter run` and
`flutter gen-l10n` in the README already, which is a better home.

The part about lib/l10n/ is just inaccurate; there's no such
directory in our tree.  I think that may have been the home of
what's now assets/l10n/, in a previous draft of the PR that
introduced this.

Author: gnprice

pubspec.yaml

@@ -80,10 +80,6 @@ dev_dependencies:
   # Keep list sorted when adding dependencies; it helps prevent merge conflicts.
 
 flutter:
-
-  # Generate localization bindings from ARB files in lib/l10n/.
-  # This happens automatically with `flutter run`
-  # but can be manually run with `flutter gen-l10n`
   generate: true
   uses-material-design: true