Revert "[vm, gc] Mark through new-space."

This reverts commit a3b7c323b055eaa948e4085877e0b4c968bf820d.

Reason for revert: We are seeing crashes in some internal testing
si_signo=Segmentation fault(11), si_code=SEGV_ACCERR(2), si_addr=0x7f1cadb85bc6
version=3.3.0-edge+google3-v2 (google3) (google3) on "linux_x64"
pid=8967, thread=8969, isolate_group=main(0x5583571aaaf0), isolate=(nil)((nil))
os=linux, arch=x64, comp=no, sim=no
isolate_instructions=7f1cad8b7e40, vm_instructions=7f1cad8b1000
fp=7f1cacc7ebc0, sp=7f1cacc7eb90, pc=5583568155bc
  pc 0x00005583568155bc fp 0x00007f1cacc7ebc0 dart::MarkingVisitorBase<true>::VisitPointers(dart::ObjectPtr*, dart::ObjectPtr*)+0x15c
  pc 0x0000558356975a1d fp 0x00007f1cacc7ec60 dart::StackFrame::VisitObjectPointers(dart::ObjectPointerVisitor*)+0x1bd
  pc 0x0000558356935c29 fp 0x00007f1cacc7ecc0 dart::UntaggedSuspendState::VisitSuspendStatePointers(dart::SuspendStatePtr, dart::ObjectPointerVisitor*)+0x89
  pc 0x0000558356815948 fp 0x00007f1cacc7ed10 dart::MarkingVisitorBase<true>::DrainMarkingStackWithPauseChecks()+0x128
  pc 0x0000558356815749 fp 0x00007f1cacc7ed80 dart::ConcurrentMarkTask::Run()+0x69

Original change's description:
> [vm, gc] Mark through new-space.
>
>  - Initial and final marking no longer visit all of new-space, reducing the STW pause for major GC.
>  - A scavenge during concurrent marking must forward / filter objects in the marking worklist that are moved / collected, increasing the STW pause for minor GC.
>  - Unreachable intergenerational cycles and weak references are collected in the next mark-sweep instead of first requiring enough scavenges to promote the whole cycle or weak target into old-space.
>  - Artificial minor GCs are no longer needed to avoid memory leaks from back-to-back major GCs.
>  - reachabilityBarrier is now just a count of major GCs.
>
> TEST=ci
> Change-Id: Ic7754e8d972763654eae2b7faa8670735d9cda3f
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/340644
> Reviewed-by: Siva Annamalai <asiva@google.com>
> Commit-Queue: Ryan Macnak <rmacnak@google.com>

Change-Id: Ice07b4eb5bef3b41c9618ef0ca7759de006ffe00
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/343060
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Siva Annamalai <asiva@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>