The ornamental design for a walk-in bathtub, as shown and described.
FIG. 1 is a top, front, left-side perspective view of a walk-in bathtub showing our new design;
FIG. 2 is a top, front, left-side perspective view thereof with a door in the open configuration;
FIG. 3 is a top, rear, left-side perspective view of the walk-in bathtub;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a right-side elevational view thereof;
FIG. 7 is a left-side elevational view thereof;
FIG. 8 is a top plan view thereof; and,
FIG. 9 is a bottom plan view thereof.
The broken lines immediately adjacent to the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environmental structure; the broken lines form no part of the claimed design.