journal-file: drop unused tail_entry_monotonic_valid field.

As pointed out by Matthijs van Duin:

https://lists.freedesktop.org/archives/systemd-devel/2018-March/040499.html

src/journal/journal-file.c

@@ -470,9 +470,6 @@ static int journal_file_refresh_header(JournalFile *f) {
         if (r < 0)
                 return r;
 
-        if (sd_id128_equal(boot_id, f->header->boot_id))
-                f->tail_entry_monotonic_valid = true;
-
         f->header->boot_id = boot_id;
 
         r = journal_file_set_online(f);
@@ -1797,8 +1794,6 @@ static int journal_file_link_entry(JournalFile *f, Object *o, uint64_t offset) {
         f->header->tail_entry_realtime = o->entry.realtime;
         f->header->tail_entry_monotonic = o->entry.monotonic;
 
-        f->tail_entry_monotonic_valid = true;
-
         /* Link up the items */
         n = journal_file_entry_n_items(o);
         for (i = 0; i < n; i++) {

src/journal/journal-file.h

@@ -90,8 +90,6 @@ typedef struct JournalFile {
         bool close_fd:1;
         bool archive:1;
 
-        bool tail_entry_monotonic_valid:1;
-
         direction_t last_direction;
         LocationType location_type;
         uint64_t last_n_entries;