Remove cross-export warning.

lib/src/model/model_element.dart

@@ -538,19 +538,6 @@ abstract class ModelElement extends Canonicalization
       };
     }).toList(growable: true);
 
-    // Avoid claiming canonicalization for elements outside of this element's
-    // defining package.
-    // TODO(jcollins-g): Make the else block unconditional.
-    if (candidateLibraries.isNotEmpty &&
-        !candidateLibraries.any((l) => l.package == definingLibrary.package)) {
-      warn(PackageWarning.reexportedPrivateApiAcrossPackages,
-          message: definingLibrary.package.fullyQualifiedName,
-          referredFrom: candidateLibraries);
-    } else {
-      candidateLibraries
-          .removeWhere((l) => l.package != definingLibrary.package);
-    }
-
     if (candidateLibraries.isEmpty) {
       return null;
     }

lib/src/model/package_graph.dart

@@ -418,7 +418,6 @@ class PackageGraph with CommentReferable, Nameable, ModelBuilder {
       PackageWarning.ambiguousDocReference ||
       PackageWarning.ignoredCanonicalFor ||
       PackageWarning.packageOrderGivesMissingPackageName ||
-      PackageWarning.reexportedPrivateApiAcrossPackages ||
       PackageWarning.unresolvedDocReference ||
       PackageWarning.unknownMacro ||
       PackageWarning.unknownHtmlFragment ||

lib/src/warnings.dart

@@ -181,13 +181,6 @@ enum PackageWarning implements Comparable<PackageWarning> {
         'The category-order flag on the command line was given the name of a '
         'nonexistent package',
   ),
-  reexportedPrivateApiAcrossPackages(
-    'reexported-private-api-across-packages',
-    'private API of {0} is reexported by libraries in other packages: ',
-    shortHelp:
-        'One or more libraries reexports private API members from outside its '
-        'own package',
-  ),
   unresolvedDocReference(
     'unresolved-doc-reference',
     'unresolved doc reference [{0}]',