Remove signer log when DKG doesn't succeed

[hstove]

When a new 2.5 reward cycle starts and the aggregate key isn't set, the signer has this log every few seconds:

Signer #N: Could not lookup current miner while in active reward cycle

I thought we'd removed this log during 2.5, but looks like we haven't.

Relates to:

• #4729

[wileyj]

#4760

@hstove is it feasible to have a PR addressing this for next week?

[diwakergupta]

Fixed by #4768

[blockstack-devops]

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