Get rid of weighted average for heap_allocatable_bytes_expand #12922

Job	Run time
Windows 11-arm/Visual C++ (btest test-basic test-tool)	6m 20s
Windows 2025/Visual C++ 2022 (test-bundled-gems)	3m 55s
Windows 2025/Visual C++ 2022 (check)	5m 32s
Windows 2022/Visual C++ 2022 (check)	4m 39s
Windows result	4s
	20m 30s

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