MINOR: checks: I/O callback function only rely on the data layer wake callback

MINOR: checks: I/O callback function only rely on the data layer wake callback

Most of code in event_srv_chk_io() function is inherited from the checks before
the recent refactoring. Now, it is enough to only call wake_srv_chk(). Since the
refactoring, the removed code is dead and never called. wake_srv_chk() may only
return 0 if tcpcheck_main() returns 0 and the check status is unknown
(CHK_RES_UNKNOWN). When this happens, nothing is performed in event_srv_chk_io().