The ornamental design for a shelfsystem, as shown and described.
FIG. 1 is a perspective view of the shelf system having a knob-enabled suction structure;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken lines showing in the figures are included for the purpose of illustrating portions of the shelf system and form no part of the claimed design. The broken lines showing immediately adjacent the shaded areas represent boundaries to the claimed design and form no part of the claimed design.