remove gc_feedback mechanism (#6863)

It's been dead-code-at-runtime for 9 months, let's remove it.
We can always re-introduce it at a later point.

Came across this while working on #6861, which will touch
`time_for_new_image_layer`. This is an opporunity to make that function
simpler.

test_runner/performance/test_gc_feedback.py

@@ -13,6 +13,11 @@ def test_gc_feedback(neon_env_builder: NeonEnvBuilder, zenbenchmark: NeonBenchma
     Information about image layers needed to collect old layers should
     be propagated by GC to compaction task which should take in in account
     when make a decision which new image layers needs to be created.
+
+    NB: this test demonstrates the problem. The source tree contained the
+    `gc_feedback` mechanism for about 9 months, but, there were problems
+    with it and it wasn't enabled at runtime.
+    This PR removed the code: https://github.com/neondatabase/neon/pull/6863
     """
     env = neon_env_builder.init_start()
     client = env.pageserver.http_client()

test_runner/regress/test_attach_tenant_config.py

@@ -166,7 +166,6 @@ def test_fully_custom_config(positive_env: NeonEnv):
             "threshold": "23h",
         },
         "evictions_low_residence_duration_metric_threshold": "2days",
-        "gc_feedback": True,
         "gc_horizon": 23 * (1024 * 1024),
         "gc_period": "2h 13m",
         "heatmap_period": "10m",