[memorygc](lrucache) do not prune memory if prune is disabled (#59478)

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Author: yiguolei

be/src/runtime/memory/lru_cache_policy.h

@@ -254,7 +254,11 @@ class LRUCachePolicy : public CachePolicy {
         if (std::dynamic_pointer_cast<doris::DummyLRUCache>(_cache)) {
             return 0;
         }
-
+        if (!_enable_prune) {
+            LOG(INFO) << "[MemoryGC] " << type_string(_type)
+                      << " cache prune disabled, so could not adjust capacity to free memory";
+            return 0;
+        }
         size_t old_capacity = get_capacity();
         int64_t old_mem_consumption = mem_consumption();
         int64_t old_usage = get_usage();