The ornamental design for a portable shower system, as shown and described.
FIG. 1 is a perspective view of the portable shower system;
FIG. 2 is a front view;
FIG. 3 is a back view;
FIG. 4 is a left side view;
FIG. 5 is a right side view;
FIG. 6 is a top view; and,
FIG. 7 is a bottom view thereof.
The broken lines shown in the bottom view are for the purpose of illustrating portions of the portable shower system and form no part of the claimed design.