[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

libunwind/src/UnwindCursor.hpp

@@ -1965,10 +1965,6 @@ bool UnwindCursor<A, R>::getInfoFromSEH(pint_t pc) {
       _info.handler = 0;
     }
   }
-#elif defined(_LIBUNWIND_TARGET_ARM)
-  _info.end_ip = _info.start_ip + unwindEntry->FunctionLength;
-  _info.lsda = 0; // FIXME
-  _info.handler = 0; // FIXME
 #endif
   setLastPC(pc);
   return true;