MAINT-2612: Remove no-longer-valid unused-var warning suppressions.

Somehow viewer-cat acquired a lot of source lines (in several different files)
of the form:
    (void)somevar;
where ?somevar? was not declared. Apparently someone introduced an unused
declaration of ?somevar?, encountered unused-variable warnings, introduced the
(void) casts to suppress those warnings, then deleted the declarations --
without deleting the (void) casts. It seems worth noting that the resulting
code should not have compiled successfully on any platform.