Revert "drivers/input/ft5x06.c: Back out part of last PR. I think that is correct to detach the interrupt handler when the drivers is unlinked."

No... I was misled.  The shutdown function is usually called when the
driver is unlinked.  But not in this case.  In this case, it is simply
called when the last reference is closed.  And no, you don't want to
detach the interrupt handler in that case.  My bad.

This reverts commit b79e6b9c9a.

drivers/input/ft5x06.c

@@ -780,13 +780,6 @@ static void ft5x06_shutdown(FAR struct ft5x06_dev_s *priv)
 
   config->clear(config);
   config->enable(config, false);
-
-  /* Detach the interrupt handler (otherwise, the attach may failed if the
-   * driver is re-registered, depending upon how the lower-half is
-   * implemented.
-   */
-
-  (void)config->attach(config, NULL, NULL);
 #endif
 }