Fix rb_gc_adjust_memory_usage double-counting

[omohokcoj]

Memory wrapper tracked wasm memory via its own ManuallyTracked, but that memory lives in the Store and is already reported to Ruby by the store's TrackingResourceLimiter causing double-counting:

require "wasmtime"

pages = 200
bytes = pages * 65_536
store = Wasmtime::Store.new(Wasmtime::Engine.new)

before = GC.stat(:malloc_increase_bytes)
Wasmtime::Memory.new(store, min_size: pages)
delta = GC.stat(:malloc_increase_bytes) - before

puts "linear memory: %d bytes | reported to GC: %d bytes | %.1fx\n" % [bytes, delta, delta.to_f / bytes]

# output:  linear memory: 13107200 bytes | reported to GC: 26216600 bytes | 2.0x - double-counting

This PR removes redundant GC stats tracking from Memory that double-counted wasm memory in Ruby (since the store's resource limiter already reports it).

[saulecabrera]

LGTM, thanks!