Remove the terminal option from config.

The welcome message is displayed using xmessage(1), not using your
terminal. Thus, it makes no sense to have this option anymore. Also,
the new lex/yacc parser cannot correctly handle the situation:
normal variables are expanded before parsing the file. As a replacement,
you can use:
  set $terminal /usr/bin/urxvt

src/config.c

@@ -597,7 +597,6 @@ void load_configuration(xcb_connection_t *conn, const char *override_configpath,
         }
         }
 
-        REQUIRED_OPTION(terminal);
         REQUIRED_OPTION(font);
 
         /* Set an empty name for every workspace which got no name */