Fix warnings: option.c: do_set(): Dead assignment: HI.

Problem    : Dead assignment @ 2566.
Diagnostic : Harmless issue.
Rationale  : `nextchar` is used as a lookahead buffer for the character
             next to the currently examined token. Sometimes it also
             saves that char while original string is modified (original
             position of nextchar is nullified for the string to
             terminate there). In summary, it's an auxiliary variable
             with no particular complex meaning. Safe to remove if not
             used.
Resolution : Remove dead assignment.

src/nvim/option.c

@@ -2563,7 +2563,6 @@ do_set (
       /* find end of name */
       key = 0;
       if (*arg == '<') {
-        nextchar = 0;
         opt_idx = -1;
         /* look out for <t_>;> */
         if (arg[1] == 't' && arg[2] == '_' && arg[3] && arg[4])