Revert "[ORC] Add a shared-memory based orc::MemoryMapper."

This reverts commit 5acd4716.

Breaks shared library build with:

  ld.lld-12: error: undefined symbol: shm_open
  >>> referenced by ExecutorSharedMemoryMapperService.cpp:68
  (/home/culrho01/llvm-project/llvm/lib/ExecutionEngine/Orc/TargetProcess/ExecutorSharedMemoryMapperService.cpp:68)
  >>>
  lib/ExecutionEngine/Orc/TargetProcess/CMakeFiles/LLVMOrcTargetProcess.dir/ExecutorSharedMemoryMapperService.cpp.o:(llvm::orc::rt_bootstrap::ExecutorSharedMemoryMapperService::reserve[abi:cxx11](unsigned
  long))
  >>> did you mean: sem_open
  >>> defined in:
  /usr/bin/../lib/gcc/aarch64-linux-gnu/9/../../../aarch64-linux-gnu/libpthread.so