Disregard and remove legacy open rounds recovered by Service

[poszu]

Fixes a problem with poet version bump.

Previously, the datadir contained both the round in-progress (E) and the open round (E+1). The new Service implemented in #365 doesn't keep open round anymore (it's handled by Registration). When bumping to v0.9.2, the Service will be confused if it finds an open round and will overwrite the round in progress. This PR mitigates it by removing the legacy open round.

[codecov]

Codecov Report

Merging #390 (f59d0e4) into develop (00a5521) will increase coverage by 0.2%.
The diff coverage is 61.5%.

@@            Coverage Diff            @@
##           develop    #390     +/-   ##
=========================================
+ Coverage     73.6%   73.9%   +0.2%     
=========================================
  Files           23      23             
  Lines         1954    1962      +8     
=========================================
+ Hits          1439    1450     +11     
+ Misses         388     386      -2     
+ Partials       127     126      -1     

Files Changed	Coverage Δ
service/round.go	84.8% <44.4%> (-0.4%)	⬇️
service/service.go	78.3% <100.0%> (+2.6%)	⬆️