Simplify HKDF interface.

libaxolotl/src/main/java/org/whispersystems/libaxolotl/kdf/HKDF.java

@@ -63,7 +63,7 @@ public abstract class HKDF {
       ByteArrayOutputStream results        = new ByteArrayOutputStream();
       int                   remainingBytes = outputSize;
 
-      for (int i= getIterationStartOffset();i<iterations + getIterationEndOffset();i++) {
+      for (int i= getIterationStartOffset();i<iterations + getIterationStartOffset();i++) {
         Mac mac = Mac.getInstance("HmacSHA256");
         mac.init(new SecretKeySpec(prk, "HmacSHA256"));
 
@@ -89,6 +89,5 @@ public abstract class HKDF {
   }
 
   protected abstract int getIterationStartOffset();
-  protected abstract int getIterationEndOffset();
 
 }

libaxolotl/src/main/java/org/whispersystems/libaxolotl/kdf/HKDFv2.java

@@ -5,9 +5,4 @@ public class HKDFv2 extends HKDF {
   protected int getIterationStartOffset() {
     return 0;
   }
-
-  @Override
-  protected int getIterationEndOffset() {
-    return 0;
-  }
 }

libaxolotl/src/main/java/org/whispersystems/libaxolotl/kdf/HKDFv3.java

@@ -5,9 +5,4 @@ public class HKDFv3 extends HKDF {
   protected int getIterationStartOffset() {
     return 1;
   }
-
-  @Override
-  protected int getIterationEndOffset() {
-    return 1;
-  }
 }