scheduler: clear previous status when validator status changes

[xenowits]

Clears previous status when validator status changes. For ex: when a validator progresses from pending_queued to active_ongoing, we currently see two values, one for each status. This PR fixes this by removing the older statuses. In this example, it removes status pending_queued.

category: refactor
ticket: ObolNetwork/charon-distributed-validator-node#139

[codecov]

Codecov Report

Base: 53.24% // Head: 53.27% // Increases project coverage by +0.02% 🎉

Coverage data is based on head (eb51505) compared to base (1439367).
Patch coverage: 76.92% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1489      +/-   ##
==========================================
+ Coverage   53.24%   53.27%   +0.02%     
==========================================
  Files         149      149              
  Lines       18979    18988       +9     
==========================================
+ Hits        10106    10116      +10     
- Misses       7489     7492       +3     
+ Partials     1384     1380       -4     

Impacted Files	Coverage Δ
core/scheduler/metrics.go	81.25% <72.72%> (-18.75%)	⬇️
core/scheduler/scheduler.go	72.66% <100.00%> (ø)
app/app.go	63.25% <0.00%> (-0.76%)	⬇️
core/qbft/qbft.go	71.81% <0.00%> (-0.63%)	⬇️
app/vmock.go	76.16% <0.00%> (+0.51%)	⬆️
core/dutydb/memory.go	69.28% <0.00%> (+0.69%)	⬆️
core/priority/prioritiser.go	65.43% <0.00%> (+2.88%)	⬆️

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[xenowits]