Delete Migrate bounty endpoint

[kevkevinpal]

This is a left over from when we migrated the bounties so we should get rid of that

[kevkevinpal]

@elraphty think I think we're good to delete this endpoint now, I can try opening up a PR for it

[cybercraftsolutionsllc]

Submitted PR: #2792

This removes the leftover /migrate_bounties endpoint from the router, deletes the obsolete MigrateBounties handler and its migration tests, and removes the endpoint from generated Swagger docs.

Validation:

• go test handlers/bounties.go -run '^$' -count=1
• go test ./routes -run '^$' -count=1
• rg -n "migrate_bounties|MigrateBounties" handlers routes docs -g "*.go" -g "*.json" -g "*.yaml" returned no references
• git diff --check

Known local blockers:

• Full ./handlers package test compile is still blocked by the existing stale mock GetBountyByUnlockCode issue.
• File-list TestGetWantedsHeader is blocked by local Postgres not running at localhost:5532.
• Full TestNewRouter hits existing nil DB feature-flag middleware/xgo test setup behavior; routes package compile with no tests passes.