Fix warnings: screen.c: win_line(): Dead assigment: HI.

Problem: Dead assigment.
         http://neovim.org/doc/reports/clang/report-7362ba.html#EndPath

Diagnostic: Harmless issue.
Rationale : `boguscols` is in fact unread by downstream code.
Resolution: Comment out. This is preferred here over just removing the
            line because involved logic is complex, and future readers
            of this code could find this extra knowledge useful to
            understand what the code is doing.

src/nvim/screen.c

@@ -3867,7 +3867,7 @@ win_line (
       /* Get rid of the boguscols now, we want to draw until the right
        * edge for 'cursorcolumn'. */
       col -= boguscols;
-      boguscols = 0;
+      // boguscols = 0;  // Disabled because value never read after this
 
       if (draw_color_col)
         draw_color_col = advance_color_col(VCOL_HLC, &color_cols);