logs-show: do not use _SOURCE_MONOTONIC_TIMESTAMP field

The timestamp is not in CLOCK_MONOTONIC, but CLOCK_BOOTTIME,
while header monotonic timestamp is in CLOCK_MONOTONIC. Hence, we cannot
adjust timestamp by comparing with header monotonic timestamp and
_SOURCE_MONOTONIC_TIMESTAMP field.

Fixes a regression caused by affde1d.
Fixes systemd#33293.

(cherry picked from commit 144498e)

Author: yuwata

src/shared/logs-show.c

@@ -450,6 +450,9 @@ static void parse_display_realtime(
         assert(j);
         assert(ret);
 
+        // FIXME: _SOURCE_MONOTONIC_TIMESTAMP is in CLOCK_BOOTTIME, hence we cannot use it for adjusting realtime.
+        source_monotonic = NULL;
+
         /* First, try _SOURCE_REALTIME_TIMESTAMP. */
         if (source_realtime && safe_atou64(source_realtime, &t) >= 0 && VALID_REALTIME(t)) {
                 *ret = t;
@@ -488,6 +491,9 @@ static void parse_display_timestamp(
         assert(ret_display_ts);
         assert(ret_boot_id);
 
+        // FIXME: _SOURCE_MONOTONIC_TIMESTAMP is in CLOCK_BOOTTIME, hence we cannot use it for adjusting realtime.
+        source_monotonic = NULL;
+
         if (source_realtime && safe_atou64(source_realtime, &t) >= 0 && VALID_REALTIME(t))
                 source_ts.realtime = t;