[llvm] [lld/ELF, gold] Remove transitionary opaque pointer flags (PR #65529)

[via llvm-commits]

aeubanks wrote:

> Should also remove the equivalent here then: https://github.com/llvm/llvm-project/blob/main/llvm/tools/gold/gold-plugin.cpp#L311

done

https://github.com/llvm/llvm-project/pull/65529