Stop returning var values in upsert response

[mangalaman93]

We are returning response to the queries in an upsert. If user needs the values of variables used in upsert, it can look at the response of the query now, instead. Related to dgraph-io/dgraph#4210. Reverts the changes that were made in #102

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This change is 

[manishrjain]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @mangalaman93, @MichaelJCompton, and @pawanrawal)

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protos/api.proto, line 66 at r1 (raw file):

	Latency latency = 3;
	// Metrics contains all metrics related to the query.
	Metrics metrics = 5;

= 4, considering that we haven't done a release since introducing these fields.