Remove kind attribute from REST containers

It seems like new REST endpoints don't include "kind" attribute. This
change removes it from the code and documentation.

Change-Id: Ia73fb92ce01461e7de140b138fc72583b00170bc

gerrit-server/src/test/java/com/google/gerrit/server/change/CommentsTest.java

@@ -205,7 +205,6 @@ public class CommentsTest {
   private static void assertComment(PatchLineComment plc, CommentInfo ci) {
     assertEquals(plc.getKey().get(), ci.id);
     assertEquals(plc.getParentUuid(), ci.inReplyTo);
-    assertEquals("gerritcodereview#comment", ci.kind);
     assertEquals(plc.getMessage(), ci.message);
     assertNotNull(ci.author);
     assertEquals(plc.getAuthor(), ci.author._id);