Remove updatePositionArgsInitialize and replReadFromMetadata from replication_coordinator_test_fixture.h

Once FCV4.0 code is removed, we can expect nodes to always require a wall time when calling initialize and readFromMetadata.

We expect to remove the requireWallTime parameter as part of SERVER-42485. Once this is done, we can remove updatePositionArgsInitialize and replReadFromMetadata and instead call initialize and replReadFromMetadata respectively.