[libc++] Remove redundant if(LIBCXX_INSTALL_LIBRARY)

The individual LIBCXX_INSTALL_(SHARED|STATIC)_LIBRARY are already
dependent on whether LIBCXX_INSTALL_LIBRARY is ON or OFF.

GitOrigin-RevId: 4bd3d16c2d629fd06a2a60e45a501f4230da9633
1 file changed