This reverts commit 2a09af7e.

Reason for revert: Breaks gcc (https://logs.chromium.org/logs/v8/buildbucket/cr-buildbucket.appspot.com/8849798091364623904/+/u/build/compile/stdout?format=raw)

Original change's description:
> [heap] Make Heap::MaxRegularHeapObjectSize an inlineable constexpr
>
> Bug: v8:11263
> Change-Id: Ia86ae814434aed9795f98d80e987a1ed10fa9a1f
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2821540
> Reviewed-by: Michael Stanton <mvstanton@chromium.org>
> Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
> Commit-Queue: Camillo Bruni <cbruni@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#73991}

Bug: v8:11263
Change-Id: Iae78ae9df689e3a4653171dc34cd5fa84afc1ba0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2831476
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/master@{#73992}