Fix imported changesets being counted twice in metrics

Created by: eseliger

When a changeset was created by a campaign, but also imported by another, we counted them twice in the metrics. This fixes it and also renders the added_to_campaign column useless, so we can drop it :)

Created by: sourcegraph-bot

Notifying subscribers in CODENOTIFY files for diff 5b809aa0acade45ca2ba36aed0778379f1f3421b...cc945c7629e789b7fb029ef3c05beb9398fe7bcd.

Notify	File(s)
@LawnGnome	enterprise/internal/campaigns/rewirer/rewirer.go enterprise/internal/campaigns/store/changesets.go internal/campaigns/changeset.go

Created by: codecov[bot]

Codecov Report

Merging #16848 (cc945c7) into main (5b809aa) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main   #16848      +/-   ##
==========================================
- Coverage   52.44%   52.44%   -0.01%     
==========================================
  Files        1705     1705              
  Lines       84796    84794       -2     
  Branches     7611     7676      +65     
==========================================
- Hits        44475    44471       -4     
- Misses      36421    36422       +1     
- Partials     3900     3901       +1     

Flag	Coverage Δ
go	51.62% <100.00%> (-0.01%)
integration	29.70% <ø> (ø)
storybook	28.76% <ø> (ø)
typescript	54.40% <ø> (ø)
unit	35.03% <ø> (ø)

Impacted Files	Coverage Δ
enterprise/internal/campaigns/store/changesets.go	83.33% <ø> (-0.10%)
internal/campaigns/changeset.go	49.61% <ø> (ø)
internal/usagestats/campaigns.go	92.98% <100.00%> (ø)
.../internal/codeintel/resolvers/graphql/locations.go	83.50% <0.00%> (-2.07%)