Fix next/prev message detection with fastRecords.

The previous way we were getting the next/previous record didn't take into
consideration that some records aren't in the cursor -- some are in the
fastRecords map. We now use the proper position to get the next/previous
message.

src/org/thoughtcrime/securesms/database/CursorRecyclerViewAdapter.java

@@ -211,13 +211,6 @@ public abstract class CursorRecyclerViewAdapter<VH extends RecyclerView.ViewHold
     return position - getFastAccessSize();
   }
 
-  protected int getRawCursorPosition(int position) {
-    if (hasHeaderView()) {
-      position += 1;
-    }
-    return position;
-  }
-
   protected int getFastAccessItemViewType(int position) {
     return 0;
   }