Revert "[BPF] Attribute preserve_static_offset for structs"

This reverts commit cb13e928.
Buildbot reports MSAN failures in tests added in this commit:
https://lab.llvm.org/buildbot/#/builders/5/builds/38806

Failing tests:
  LLVM :: CodeGen/BPF/preserve-static-offset/load-arr-pai.ll
  LLVM :: CodeGen/BPF/preserve-static-offset/load-ptr-pai.ll
  LLVM :: CodeGen/BPF/preserve-static-offset/load-struct-pai.ll
  LLVM :: CodeGen/BPF/preserve-static-offset/load-union-pai.ll
  LLVM :: CodeGen/BPF/preserve-static-offset/store-pai.ll