Remove deprecated Strings class.

runtime/lib/string_patch.dart

Index: runtime/lib/string_patch.dart
diff --git a/runtime/lib/string_patch.dart b/runtime/lib/string_patch.dart
index 7157ff9a13acf736c68607440989e10bd0bcc209..6a9cf5497e5da5d3f3140c2b89f0cc9030ba0bf4 100644
--- a/runtime/lib/string_patch.dart
+++ b/runtime/lib/string_patch.dart
@@ -7,13 +7,3 @@ patch class String {
     return _StringBase.createFromCharCodes(charCodes);
   }
 }
-
-patch class Strings {
-  /* patch */ static String join(Iterable<String> strings, String separator) {
-    return _StringBase.join(strings, separator);
-  }
-
-  /* patch */ static String concatAll(List<String> strings) {
-    return _StringBase.concatAll(strings);
-  }
-}