The ornamental design for a wall insert, as shown and described.
FIG. 1 is a front view of the wall insert in accordance with the present invention, showing it installed in a hallway of an aircraft, the hallway walls being shown by phantom lines and form no part of the claimed design;
FIG. 2 is a top view of the wall insert shown in FIG. 1;
FIG. 3 is a rear view of the wall insert shown in FIG. 1, and without the hallway walls shown;
FIG. 4 is a side view of the wall insert shown in FIG. 1, also without the hallways shown; and,
FIG. 5 is a perspective of the wall insert shown in FIG. 1.