remove references to 'Cursor.clipIdx' and delete that function once it is no longer used

fcore/cursor.sml

@@ -1251,38 +1251,4 @@ struct
           end
       | [] => true
     end
-
-  (* Prerequisite: lineGap is moved to cursorIdx *)
-  fun clipIdx (lineGap: LineGap.t, cursorIdx) =
-    let
-      val {rightStrings, idx = bufferIdx, ...} = lineGap
-    in
-      (* We are trying to check if cursorIdx is within the buffer. *)
-      case rightStrings of
-        _ :: _ :: _ =>
-          (* if there is a string after the hd, 
-           * we are definitely in a valid idx and should return it *)
-          cursorIdx
-      | [hd] =>
-          let
-            val strIdx = cursorIdx - bufferIdx
-          in
-            if strIdx < String.size hd - 1 then
-              (* if we are before the last char in the string.
-               * Unix file endings always have \n at the end
-               * but we do not want cursor to ever go to end
-               * as vi also does not go to the very end.
-               * This is why we check strIdx is before the last char. 
-               * *)
-              cursorIdx
-            else
-              (* if end of buffer - 2 is greater than 0, then that;
-               * else, 0 *)
-              Int.max (bufferIdx + String.size hd - 2, 0)
-          end
-      | [] =>
-          (* if end of buffer - 2 is greater than 0, then that;
-           * else, 0 *)
-          Int.max (bufferIdx - 2, 0)
-    end
 end