Drop xcb_flush() before xcb_aux_sync() (#4378)

xcb_flush() flushes xcb's output buffer. Thus, after this call, all
previous requests are surely written to the connection with the X11
server.

xcb_aux_sync() synchronises with the X11 server. It makes sure all
previous requests were sent to the X11 server and already processed. It
does this via free(xcb_get_input_focus_reply(xcb_get_input_focus())): It
sends a request and waits for its reply. It is guaranteed that the X11
server processes requests in-order.

This means that the sequence xcb_flush(); xcb_aux_sync() first writes
whatever is in the output buffer and then does another write for the
four bytes of the GetInputFocus request from xcb_aux_sync().

Put differently: xcb_flush(); xcb_aux_sync() doesn't do anything more
than just xcb_aux_sync(). In fact, it has slightly more overhead for the
same result.

Thus, this commit drops all calls to xcb_flush() immediately after
xcb_aux_sync().

Signed-off-by: Uli Schlachter <psychon@znc.in>

i3bar/src/xcb.c

@@ -1532,7 +1532,6 @@ void clean_xcb(void) {
     free_font();
 
     xcb_free_cursor(xcb_connection, cursor);
-    xcb_flush(xcb_connection);
     xcb_aux_sync(xcb_connection);
     xcb_disconnect(xcb_connection);