[llvm-objcopy] Delete redundant !Config.xx.empty() when followed by positive is_contained() check

Summary: The original intention of !Config.xx.empty() was probably to emphasize the thing that is currently considered, but I feel the simplified form is actually easier to understand and it is also consistent with the call sites in other llvm components.

Reviewers: alexshap, rupprecht, jakehehrlich, jhenderson, espindola

Reviewed By: alexshap, rupprecht

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D55040

llvm-svn: 347891