Fix warnings: screen.c: draw_tabline(): Dead assignment: HI.

Problem: Dead assignment @ 7711.
         http://neovim.org/doc/reports/clang/report-835eb6.html#EndPath

Diagnostic: Harmless issue.
Rationale : `scol` is only used within `FOR_ALL_TABS` body, which
            assigns another value to `scol` at the beginning of each
            iteration. If `FOR_ALL_TABS` body would not execute (no
            tabs) nothing harmful would happen, as code following
            `FOR_ALL_TABS` doesn't use `scol`.
Resolution: Remove.

src/nvim/screen.c

@@ -7709,7 +7709,6 @@ static void draw_tabline(void)
 
     attr = attr_nosel;
     tabcount = 0;
-    scol = 0;
 
     FOR_ALL_TABS(tp) {
       if (col >= Columns - 4) {