tree-wide: drop signal.h when signal-util.h is included

src/shared/ask-password-api.c

@@ -5,7 +5,6 @@
 #include <inttypes.h>
 #include <limits.h>
 #include <poll.h>
-#include <signal.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>

src/shared/pager.c

@@ -1,7 +1,6 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include <errno.h>
-#include <signal.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdio.h>