The ornamental design for a backlesshat, as shown and described.
FIG. 1 is a rear perspective view of a backless hat illustrating my new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a right side view thereof;
FIG. 4 is a left side view thereof; and,
FIG. 5 is a top view thereof.
The ventilation holes outlined by broken lines within the gore panels of the backless hat form no part of the claimed design. While the remaining broken lines along the crown gore panels and brim of the backless hat depict features of the backless hat that form no part of the claimed design.