CLEANUUP: checks: don't set conn->handle.fd to -1

This used to be needed to know whether there was a check in progress a
long time ago (before tcp_checks) but this is not true anymore and even
becomes wrong after the check is reused as conn_init() initializes it
to DEAD_FD_MAGIC.

src/checks.c

@@ -2986,8 +2986,6 @@ const char *init_check(struct check *check, int type)
 		return "out of memory while allocating check connection";
 	}
 
-	check->conn->handle.fd = -1; /* no agent in progress yet */
-
 	return NULL;
 }