The unpoison was added in the initial commit 1.7.2-68-gafd229393 that
supported ASAN. It is not clear why do we need it as we don't poison
stack memory manually.

Part of #7327

NO_TEST=removing unfunctional code
NO_CHANGELOG=removing unfunctional code
NO_DOC=removing unfunctional code

(cherry picked from commit 0784f7b7)