chore: remove unifiedStorageSearchSprinkles feature flag
The feature flag is no longer needed because:
- OSS: usageinsights code doesn't exist in OSS builds
- Enterprise On-Prem: uses local SQL storage when enable_search=true
- Cloud: explicitly configures sprinkles_api_server URL
The sprinkles functionality now works automatically based on:
- enable_search config (enforced true for unified storage mode 5)
- sprinkles_api_server config (empty = local storage, set = remote API)