Align bevy_transform's feature flags

bevy_transform no longer depends on bevy_log. The bevy_log feature flag has been removed.

Tracing instrumentation is now gated on the new trace feature (using tracing directly, matching bevy_ecs):

# 0.18
bevy_transform = { features = ["bevy_log"] }

# 0.19
bevy_transform = { features = ["trace"] }

Parallel transform propagation is no longer tied to the std feature. It now requires the explicit multi_threaded feature:

# 0.18 — parallel was enabled implicitly via std
bevy_transform = { features = ["std"] }

# 0.19 — opt in explicitly
bevy_transform = { features = ["std", "multi_threaded"] }