Remove unrecommended flag from output:

The current output indirectly advertizes premature out-of-order migration,
which would be bad if it happened widely.

We can keep the flag, but should not advertize it.

Change-Id: I8bde10489cef8e1104e2e5216e03aed61bc95ba0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153612
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Kevin Moore <kevmoo@google.com>
Commit-Queue: Michael Thomsen <mit@google.com>

Author: mit-mit

pkg/nnbd_migration/lib/migration_cli.dart

@@ -168,9 +168,6 @@ class DependencyChecker {
       _logger.stderr('');
       _logger.stderr('Visit https://dart.dev/tools/pub/cmd/pub-outdated for '
           'more information.');
-      _logger.stderr('');
-      _logger.stderr('You can force migration with '
-          "'--${CommandLineOptions.skipPubOutdatedFlag}' (not recommended).");
       return false;
     }
     return true;