kernel/vm_manager: Remove unnecessary heap_used data member

This isn't required anymore, as all the kernel ever queries is the size
of the current heap, not the total usage of it.

src/core/hle/kernel/svc.cpp

@@ -806,7 +806,7 @@ static ResultCode GetInfo(u64* result, u64 info_id, u64 handle, u64 info_sub_id)
             return RESULT_SUCCESS;
 
         case GetInfoType::TotalHeapUsage:
-            *result = process->VMManager().GetTotalHeapUsage();
+            *result = process->VMManager().GetCurrentHeapSize();
             return RESULT_SUCCESS;
 
         case GetInfoType::IsVirtualAddressMemoryEnabled: