The ornamental design for a bitless rein assembly, as shown and described.
FIG. 1 is a pictorial top plan view of a bitless rein assembly showing my new design, the broken lines are for illustrative purposes and form no part of the claimed invention; and,
FIG. 2 is a perspective view of the bitless rein assembly of FIG. 1 in use, the broken lines are for illustrative purposes and form no part of the claimed invention.