[ExecutionEngine] Remove unused functions

This patch removes:

  cloneModuleFlagsMetadata
  moveFunctionBody
  moveGlobalVariableInitializer

Their last uses were removed by:

  commit 6154c411
  Author: Lang Hames <lhames@gmail.com>
  Date:   Mon Sep 7 21:21:28 2020 -0700

Differential Revision: https://reviews.llvm.org/D152668