option: use char* for get_option_value() param

'name' param is casted to char_u* within get_option_value().
Most calls to get_option_value() cast arg to 'name' from char to char_u.
Remove these pointless type casts.

src/nvim/spell.c

@@ -6624,7 +6624,7 @@ void ex_spelldump(exarg_T *eap)
   if (no_spell_checking(curwin)) {
     return;
   }
-  get_option_value((char_u *)"spl", &dummy, &spl, OPT_LOCAL);
+  get_option_value("spl", &dummy, &spl, OPT_LOCAL);
 
   // Create a new empty buffer in a new window.
   do_cmdline_cmd("new");