This reverts commit fdabcadd.

This was already enabled by-default in AOT mode in [0] - which made the
gen_snapshot invocations use "--lazy-async-stacks --no-causal-async-stacks".

See go/dart-10x-faster-async for more information.

[0] https://github.com/flutter/flutter/commit/347823234fd

This was only necessary when the Engine had to build in multiple buildroots
where the sources where checked out at different paths relative to the
buildroot. This is no longer the case and there are already cases GN rules
have been written that mix and match variable usage with the direct
specification of the path to the Flutter sources relative to the sole buildroot.

This reverts commit f20c773c.

This reverts commit e78c3f59.

This is part of the effort to port over dart_aot_runner