Remove kPrefsDynamicPartitionMetadataInitialized.

DeltaPerformer::InitPartitionMetadata() should always happen when
populating the InstallPlan on each update attempt. Persisting the value
across attempts would wrongly skip the initialization on resuming.

Bug: 120465293
Test: Cancel and resume an update that uses dynamic partitions.
Change-Id: Ia41c65bc26aa438f92edf5a8dff08d691afa64db