[llvm] [lld/ELF] Remove transitionary -plugin-opt=opaque-pointers flag (PR #65529)

[via llvm-commits]

aeubanks wrote:

> Okay to remove the option. Wasn't there some tests using it?

I believe they were removed when the no-opaque-pointers versions of the flags were removed

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