Revert D157750 "[Driver][CodeGen] Properly handle -fsplit-machine-functions...

Revert D157750 "[Driver][CodeGen] Properly handle -fsplit-machine-functions for fatbinary compilation."

This reverts commit 317a0fe5.
This reverts commit 30c4b97a.

See post-commit discussions on https://reviews.llvm.org/D157750 that
we should use a different mechanism to handle the error with --cuda-gpu-arch=

The IR/DiagnosticInfo.cpp, warn_drv_for_elf_only, codegne tests in
clang/test/Driver, and the following driver behavior (downgrading error
to warning) changes are undesired.
```
% clang --target=riscv64 -fsplit-machine-functions -c a.c
warning: -fsplit-machine-functions is not valid for riscv64 [-Wbackend-plugin]
```