Revert "Enable strict gcstress unwind checks."

This reverts commit 022f9e555bbad95b3e8699731ca5455151dfca8a.

Reason for revert: Strict unwind checks are breaking ART run-test
`1963-add-to-dex-classloader-in-memory` in gcstress mode; disable
them until a proper fix is found for this test failure.

Test: Run `1963-add-to-dex-classloader-in-memory` with gcstress
Bug: 190805741
Change-Id: Ic9afab0e5f8571c75c8bde8270a7655a6343c35e