[libunwind] Remove unused ARM SEH placeholder code

There's no such corresponding code for ARM64 (which has been working
in production for years). The SEH version of the Unwind functions
(e.g. `_Unwind_GetLanguageSpecificData`) doesn't use these fields.

The `_Unwind_ForcedUnwind` function would need these bits though,
but that's not used in normal C++ exception unwinding.

Differential Revision: https://reviews.llvm.org/D126868