GNU: Remove non-functional code in OBJCXX support In commit 9e66397c28 (Languages: Add support for Objective-C++, 2019-09-13, v3.16.0-rc1~44^2~2) we set `CMAKE_OBCXX_LINK_FLAGS`. It was a typo for `CMAKE_OBJCXX_LINK_FLAGS`, and so has not had any effect.