Remove max heap size for Julia-style GC trigger

[qinsoon]

mmtk-julia/mmtk/src/gc_trigger.rs

Lines 263 to 267 in 4a348c0

	/// Is current heap full?
	fn is_heap_full(&self, plan: &dyn Plan<VM = JuliaVM>) -> bool {
	plan.get_reserved_pages()
	>= conversions::bytes_to_pages_up(self.max_total_memory.load(Ordering::Relaxed))
	}

mmtk-julia/mmtk/src/gc_trigger.rs

Lines 274 to 277 in 4a348c0

	/// Return the upper bound of heap size
	fn get_max_heap_size_in_pages(&self) -> usize {
	conversions::bytes_to_pages_up(self.max_total_memory.load(Ordering::Relaxed))
	}

These lines still uses max_total_memory as a hard limit for MMTk, while Julia treats max_total_memory as a soft limit. So we may still see MMTk OOM. We should simply use usize::MAX in those lines.