[CMake] Remove variable reference that isn't used.

Summary:
This variable is never defined, so its value is always empty. Since
`libunwind` is needed to build the C++ ABI library in the first place,
it should never be linked to the C++ ABI library anyway.

Reviewers: mstorsjo, rnk

Subscribers: mgorny, christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D51644

llvm-svn: 341388