[SPARK-17089][DOCS] Remove api doc link for mapReduceTriplets operator

## What changes were proposed in this pull request?

Remove the api doc link for mapReduceTriplets operator because in latest api they are remove so when user link to that api they will not get mapReduceTriplets there so its more good to remove than confuse the user.

## How was this patch tested?
Run all the test cases

![screenshot from 2016-08-16 23-08-25](https://cloud.githubusercontent.com/assets/8075390/17709393/8cfbf75a-6406-11e6-98e6-38f7b319d833.png)

Author: sandy <phalodi@gmail.com>

Closes #14669 from phalodi/SPARK-17089.