Remove support of Xdbe

--dbe does not seem to have an effect on tearing, as noted in the old
manpage, and confirmed by my own testing.

So remove it, since it complicates the logic of painting.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/config_libconfig.c

@@ -297,8 +297,6 @@ parse_config(session_t *ps, struct options_tmp *pcfgtmp) {
     exit(1);
   // --alpha-step
   config_lookup_float(&cfg, "alpha-step", &ps->o.alpha_step);
-  // --dbe
-  lcfg_lookup_bool(&cfg, "dbe", &ps->o.dbe);
   // --paint-on-overlay
   lcfg_lookup_bool(&cfg, "paint-on-overlay", &ps->o.paint_on_overlay);
   // --sw-opti