dont set n.ops map entries to nil. Instead just delete them

[parasssh]

A panic was caused because n.ops map entry was set to nil in some cases.
Fix is to not set n.ops map entries to nil. Instead, just delete them.
Fixed DGRAPH-1573

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This change is 

[jarifibrahim]

The change looks good but can we please add a test for this? I understand we don't have any tests for it currently but without tests it's very hard to determine if we've fixed the issue or not.

Reviewable status: 0 of 1 files reviewed, all discussions resolved (waiting on @manishrjain and @vvbalaji-dgraph)

[martinmr]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain, @parasssh, and @vvbalaji-dgraph)

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worker/draft.go, line 140 at r1 (raw file):

				return nil, errors.Errorf("another restore operation is already running")
			}
			// remove from map and signal the closer to cancel the operation.

minor: uppercase

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worker/draft.go, line 147 at r1 (raw file):

		for otherId, otherCloser := range n.ops {
			if otherId == opRollup {
				// remove from map and signal the closer to cancel the operation.

minor: uppercase

[ashish-goswami]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain, @parasssh, and @vvbalaji-dgraph)

[jarifibrahim]

I had a chat with @parasssh and we concluded that adding test isn't straight forward.

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @manishrjain, @parasssh, and @vvbalaji-dgraph)