CMakeLists: Don't repeatedly include MARL_OS_LIBS
CMakeLists: Don't repeatedly include MARL_OS_LIBS

`target_link_libraries()` "Specify libraries or flags to use when linking a given target and/or its dependents."

Given that `target_link_libraries()` is called with `MARL_OS_LIBS` for the `marl` target, we don't need to include these libs for the tests and examples.

This completely contradicts what is observed in #60.

google/marl,google/marl,google/marl,google/marl