chore: remove legacy page-view migration code

[alexs-mparticle]

Summary

Removes migrateLegacyPageViewStorage and the LEGACY_PAGE_VIEWS_KEY constant. The migration has been live since v1.33.2; migration_retry errors are < 1% of failures as of 2026-08-19. No active sessions should have the legacy mpPageViews key remaining.

• Deleted migrateLegacyPageViewStorage export and its internal call inside loadPageViews in src/pageViewStorage.ts
• Deleted LEGACY_PAGE_VIEWS_KEY constant and removed now-unused readJSON/removeKey imports
• Removed the standalone migrateLegacyPageViewStorage(this.loggingService) call on the session-end path in src/Rokt-Kit.ts
• Dropped all migrateLegacyPageViewStorage tests and the "migrates legacy key before reading" test from test/src/pageViewStorage.spec.ts
• loadPageViews no longer takes a loggingService argument (the only use for it was the migration log); callers updated

Test plan

• Lint and build pass
• Monitor Datadog for migration_retry errors after this ships — should be zero

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