pageserver: remove legacy TimelineInfo::latest_gc_cutoff field (2/2) (#11136)

## Problem

This field was retained for backward compat only in #10707.

Once https://github.com/neondatabase/cloud/pull/25233 is released,
nothing will be reading this field.

Related: https://github.com/neondatabase/cloud/issues/24250

## Summary of changes

- Remove TimelineInfo::latest_gc_cutoff_lsn

pageserver/src/http/routes.rs

@@ -445,6 +445,9 @@ async fn build_timeline_info_common(
 
     let (pitr_history_size, within_ancestor_pitr) = timeline.get_pitr_history_stats();
 
+    // Externally, expose the lowest LSN that can be used to create a branch.
+    // Internally we distinguish between the planned GC cutoff (PITR point) and the "applied" GC cutoff (where we
+    // actually trimmed data to), which can pass each other when PITR is changed.
     let min_readable_lsn = std::cmp::max(
         timeline.get_gc_cutoff_lsn(),
         *timeline.get_applied_gc_cutoff_lsn(),
@@ -461,7 +464,6 @@ async fn build_timeline_info_common(
         initdb_lsn,
         last_record_lsn,
         prev_record_lsn: Some(timeline.get_prev_record_lsn()),
-        _unused: Default::default(), // Unused, for legacy decode only
         min_readable_lsn,
         applied_gc_cutoff_lsn: *timeline.get_applied_gc_cutoff_lsn(),
         current_logical_size: current_logical_size.size_dont_care_about_accuracy(),