Fix ExportMitsuba init for Blender 5.0 compatibility

[evanofficial]

Summary

Fixes the TypeError: ExportMitsuba.__init__() takes 1 positional argument but 2 were given error when exporting in Blender 5.0+.

Blender 5.0 changed how operator constructors work — Operator.__init__() no longer accepts arbitrary positional/keyword arguments. The *args, **kwargs in the ExportMitsuba.__init__ signature caused the mismatch. Since Operator.__init__ has always only taken self, the varargs were unnecessary.

Fixes #146