[SPARK-4950] Delete obsolete mapReduceTripelets used in Pregel

[maropu]

No description provided.

[maropu]

Any reason not to replace the api along with SPARK-3936?

[ankurdave]

We wanted to retain binary compatibility for the Pregel API, which prevented adding the TripletFields parameter. Instead it might be better to add a second version of the Pregel API with several changes: manually-specified TripletFields, aggregateMessages-style API, and custom vertex activeness (#1217).

[maropu]

Understood. I got back to the old Pregel API.
And also, I'll check #1217 later :))

[sryza]

@maropu mind including [GRAPHX] in the title so that this can get sorted properly?

[AmplabJenkins]

Can one of the admins verify this patch?

[andrewor14]

@maropu @ankurdave is this patch still relevant? If not, I would prefer to close it.

[ankurdave]

I think we can close this PR.