Replace vim_isspace() with ascii_isspace() defined in ascii.h

src/nvim/strings.c

@@ -449,16 +449,6 @@ char_u *vim_strrchr(const char_u *string, int c)
   return (char_u *) retval;
 }
 
-/*
- * Vim has its own isspace() function, because on some machines isspace()
- * can't handle characters above 128.
- */
-bool vim_isspace(int x)
-  FUNC_ATTR_CONST
-{
-  return (x >= 9 && x <= 13) || x == ' ';
-}
-
 /*
  * Sort an array of strings.
  */