[libc++abi] Remove redundant link flags on Apple platforms
These flags are already set when we create the cxxabi_shared target
using the SOVERSION and VERSION target properties, and the install_name
was already being overriden to '@rpath/libc++abi.1.dylib' by CMake
because no 'CMAKE_INSTALL_NAME_DIR' option was specified. So this is
effectively a removal of dead code with no intended functionality change.
The only think we're losing here is that we used to link against
libSystem.B.dylib instead of libSystem.dylib when building libc++abi
for macOS 10.6 -- however, I strongly suspect nobody's building
libc++abi from source for that target anymore.