Remove 6.1.5 version constant

The assumption here is that we will no longer be making a release from
the 6.1 branch. Since we assume that all versions on this branch are
actually released, we do not want to leave behind any versions that
would require a snapshot build. We do have a test that verifies that all
released versions are present here, so if another release is performed
from the 6.1 branch, that test will fail and we will know to add the
version constant at that time.

server/src/main/java/org/elasticsearch/Version.java

@@ -149,8 +149,6 @@ public class Version implements Comparable<Version> {
     public static final Version V_6_1_3 = new Version(V_6_1_3_ID, org.apache.lucene.util.Version.LUCENE_7_1_0);
     public static final int V_6_1_4_ID = 6010499;
     public static final Version V_6_1_4 = new Version(V_6_1_4_ID, org.apache.lucene.util.Version.LUCENE_7_1_0);
-    public static final int V_6_1_5_ID = 6010599;
-    public static final Version V_6_1_5 = new Version(V_6_1_5_ID, org.apache.lucene.util.Version.LUCENE_7_1_0);
     public static final int V_6_2_0_ID = 6020099;
     public static final Version V_6_2_0 = new Version(V_6_2_0_ID, org.apache.lucene.util.Version.LUCENE_7_2_1);
     public static final int V_6_2_1_ID = 6020199;
@@ -193,8 +191,6 @@ public class Version implements Comparable<Version> {
                 return V_6_2_1;
             case V_6_2_0_ID:
                 return V_6_2_0;
-            case V_6_1_5_ID:
-                return V_6_1_5;
             case V_6_1_4_ID:
                 return V_6_1_4;
             case V_6_1_3_ID: