journal-remote: do not send _BOOT_ID twice

Fixes #9090.

Reproducer:
systemd-journal-remote --split-mode=none -o /tmp/msg6.journal --trust=all --listen-http=8080
systemd-journal-upload -u http://localhost:8080
journalctl --file /tmp/msg6.journal -o verbose -n1

src/journal-remote/journal-upload-journal.c

@@ -139,6 +139,11 @@ static ssize_t write_entry(char *buf, size_t size, Uploader *u) {
                                 continue;
                         }
 
+                        /* We already printed the boot id from the data in
+                         * the header, hence let's suppress it here */
+                        if (u->field_length >= 9 && memcmp(u->field_data, "_BOOT_ID=", 9) == 0)
+                                continue;
+
                         if (!utf8_is_printable_newline(u->field_data,
                                                        u->field_length, false)) {
                                 u->entry_state = ENTRY_BINARY_FIELD_START;