pspp-sheet-view: Remove write-only variable from pspp_sheet_view_draw_bin().

Commit 9682fa965 (fixed scrolling of edited cell - fixes bug #45113)
removed the only code that uses the variable of 'has_special_cell' from
pspp_sheet_view_draw_bin(), but retained the code that set its value.
This commit removes the variable entirely.

This fixes a warning reported by GCC 4.9.1.