[lld] Remove shadowing "size" field from classes that inherit from SyntheticSection (PR #166323)

[Alexander Richardson via llvm-commits]

https://github.com/arichardson approved this pull request.

Since all of the ci checks pass this LGTM but I'd wait for @MaskRay approval before merging since I am not sure why this extra field was added initially. 

https://github.com/llvm/llvm-project/pull/166323