Revert "[docs] UBSan: Mention that print_stacktrace=1 is unsupported on Darwin"

This reverts commit r300295.

It's no longer true, print_stacktrace=1 is supported on Darwin/Windows
as of r301839.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@301960 91177308-0d34-0410-b5e6-96231b3b80d8