The ornamental design for a flushvalve, as shown and described.
FIG. 1 is a perspective view of a flush valve showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a right side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines shown in in the figures are included for the purpose of illustrating portions of the flush valve and form no part of the claimed design.