Revert "runtime: redo heap bitmap"

Revert "runtime: redo heap bitmap"

This reverts commit b589208c8cc6e08239868f47e12c1449cd797bac.

Reason for revert: Bug somewhere in this code, causing wasm and maybe linux/386 to fail.

Change-Id: I5e1e501d839584e0219271bb937e94348f83c11f
Reviewed-on: https://go-review.googlesource.com/c/go/+/422395
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>