🔥 Remove no-op posthog OS bootstrap plugin

$initial_os flag targeting was removed from the pricing-page-tts-sample
desktop/mobile experiments — it broke for anonymous traffic under
person_profiles: 'identified_only'. The plugin tried to seed $initial_os
via setPersonPropertiesForFlags but read get_property('$initial_os'),
which is empty for the exact anonymous users it targeted, so it was a
chicken-and-egg no-op. Desktop/mobile split is enforced client-side by
viewport (useDesktopScreen), not by flag conditions.

Author: williamchong