feat(pageserver): create image layers at L0-L1 boundary by default (#12669)

## Problem

Post LKB-198 rollout. We added a new strategy to generate image layers
at the L0-L1 boundary instead of the latest LSN to ensure too many L0
layers do not trigger image layer creation.

## Summary of changes

We already rolled it out to all users so we can remove the feature flag
now.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

pageserver/src/tenant/timeline.rs

@@ -448,6 +448,7 @@ pub struct Timeline {
     /// A channel to send async requests to prepare a basebackup for the basebackup cache.
     basebackup_cache: Arc<BasebackupCache>,
 
+    #[expect(dead_code)]
     feature_resolver: Arc<TenantFeatureResolver>,
 }
 

pageserver/src/tenant/timeline/compaction.rs

@@ -1326,13 +1326,7 @@ impl Timeline {
                 .max()
         };
 
-        let (partition_mode, partition_lsn) = if cfg!(test)
-            || cfg!(feature = "testing")
-            || self
-                .feature_resolver
-                .evaluate_boolean("image-compaction-boundary")
-                .is_ok()
-        {
+        let (partition_mode, partition_lsn) = {
             let last_repartition_lsn = self.partitioning.read().1;
             let lsn = match l0_l1_boundary_lsn {
                 Some(boundary) => gc_cutoff
@@ -1348,8 +1342,6 @@ impl Timeline {
             } else {
                 ("l0_l1_boundary", lsn)
             }
-        } else {
-            ("latest_record", self.get_last_record_lsn())
         };
 
         // 2. Repartition and create image layers if necessary