Revert "core/namespace: ignore ENOENT for /proc/sys/kernel/domainname and hostname"

This reverts commit 0ebc9f23fa.

With the previous commit, these files should always exist.

Closes #17979.

src/core/namespace.c

@@ -1692,12 +1692,10 @@ int setup_namespace(
                         *(m++) = (MountEntry) {
                                 .path_const = "/proc/sys/kernel/hostname",
                                 .mode = READONLY,
-                                .ignore = true,
                         };
                         *(m++) = (MountEntry) {
                                 .path_const = "/proc/sys/kernel/domainname",
                                 .mode = READONLY,
-                                .ignore = true,
                         };
                 }