Assume xcb_cursor_context_new never fails (#3955)

According to libxcb-cursor code, the only condition in which
xcb_cursor_context_new() returns a non-zero result is a memory
allocation failure[1].  Thus, it is safe to assume that
xcursor_supported is always true, and remove dead code.

[1]: https://gitlab.freedesktop.org/xorg/lib/libxcb-cursor/blob/0.1.3/cursor/cursor.c#L131-132

src/drag.c

@@ -175,7 +175,7 @@ drag_result_t drag_pointer(Con *con, const xcb_button_press_event_t *event,
                            xcb_window_t confine_to, int cursor,
                            bool use_threshold, callback_t callback,
                            const void *extra) {
-    xcb_cursor_t xcursor = (cursor && xcursor_supported) ? xcursor_get_cursor(cursor) : XCB_NONE;
+    xcb_cursor_t xcursor = cursor ? xcursor_get_cursor(cursor) : XCB_NONE;
 
     /* Grab the pointer */
     xcb_grab_pointer_cookie_t cookie;