gold-plugin: Do not set codegen opt level based on LTO opt level.

The LTO opt level should not affect the codegen opt level, and indeed
it does not affect it in lld. Ideally the codegen opt level should
be controlled by an IR-level attribute based on the compile-time opt
level, but that hasn't been implemented yet.

Differential Revision: https://reviews.llvm.org/D43040

llvm-svn: 324557