The ornamental design for a drop-in sink, as shown and described.
FIG. 1 is a perspective view from above and in front of the invention;
FIG. 2 is a top view of the invention;
FIG. 3 is a front view of the invention, the back view being identical;
FIG. 4 is a side view of the invention, the other side view being a mirror image thereof; and,
FIG. 5 is a perspective view from above and behind of the invention.
The broken lines showing portions of the drop-in sink are included for the purpose of showing the environmental structure and form no part of the claimed design.