go/worker/registration: Fix RequestShutdown for nodes with no registration

[jberci]

In cases where the registration worker was started with a dummy
goroutine that just waits for the service to stop, deregistration
requests would hang, because the dummy wasn't watching the
deregistration channel.

Closes #3524

[codecov]

Codecov Report

Merging #3721 (1ef8d88) into master (94e6981) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3721      +/-   ##
==========================================
+ Coverage   66.66%   66.84%   +0.18%     
==========================================
  Files         400      400              
  Lines       39778    39783       +5     
==========================================
+ Hits        26519    26594      +75     
+ Misses       9474     9419      -55     
+ Partials     3785     3770      -15     

Impacted Files	Coverage Δ
go/worker/registration/worker.go	67.77% <100.00%> (-0.08%)	⬇️
go/consensus/tendermint/full/services.go	80.00% <0.00%> (-4.17%)	⬇️
go/runtime/nodes/versioned.go	73.33% <0.00%> (-1.91%)	⬇️
go/common/sgx/aesm/aesm.go	69.15% <0.00%> (ø)
go/runtime/host/sgx/sgx.go	71.42% <0.00%> (ø)
go/worker/compute/executor/committee/node.go	68.55% <0.00%> (+0.11%)	⬆️
...o/consensus/tendermint/apps/beacon/backend_pvss.go	60.00% <0.00%> (+0.43%)	⬆️
...consensus/tendermint/apps/registry/transactions.go	57.20% <0.00%> (+0.44%)	⬆️
...o/consensus/tendermint/apps/scheduler/scheduler.go	72.75% <0.00%> (+0.51%)	⬆️
... and 15 more

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