[libunwind] Delete unused codeOffsetAtStackDecrement/registersInOtherRegisters/sameValueUsed

ld64 uses them to create compact unwind from DWARF call frame information.
When the code was ported to libunwind, the variables were not deleted.

Reviewed By: #libunwind, compnerd

Differential Revision: https://reviews.llvm.org/D91039