Fix another clang-tidy warning (issue #5, part 1).

See https://github.com/fltk/fltk/issues/5

As Manolo pointed out the questionable variable was not used (read)
in this case of the switch statement, hence it can be removed.

src/drivers/X11/Fl_X11_Screen_Driver.cxx

@@ -968,10 +968,8 @@ Fl_RGB_Image *Fl_X11_Screen_Driver::read_win_rectangle(int X, int Y, int w, int
             line_ptr[2] = cvals[i][2];
             
             if (index_shift > 0) {
-              index_mask >>= 2;
               index_shift -= 2;
             } else {
-              index_mask  = 192;
               index_shift = 6;
               pixel ++;
             }