core: cleanup unused variables

Leftovers from previous implementation of MountImages feature, unused now

src/core/namespace.c

@@ -73,7 +73,6 @@ typedef struct MountEntry {
         char *options_malloc;
         unsigned long flags;      /* Mount flags used by EMPTY_DIR and TMPFS. Do not include MS_RDONLY here, but please use read_only. */
         unsigned n_followed;
-        LIST_FIELDS(MountEntry, mount_entry);
 } MountEntry;
 
 /* If MountAPIVFS= is used, let's mount /sys and /proc into the it, but only as a fallback if the user hasn't mounted

src/core/namespace.h

@@ -9,7 +9,6 @@ typedef struct NamespaceInfo NamespaceInfo;
 typedef struct BindMount BindMount;
 typedef struct TemporaryFileSystem TemporaryFileSystem;
 typedef struct MountImage MountImage;
-typedef struct MountEntry MountEntry;
 
 #include <stdbool.h>