[libc++] Remove stray CMake install step for modulemap file (#89394)

The modulemap file is not generated anymore, so it's just part of our
list of includes and gets installed like every other header. We don't
need a special step to install it anymore. This was overlooked when I
removed the generation of the modulemap file.

GitOrigin-RevId: c61f0a8e94004b05d9ec115d3bff8cff331b4491
1 file changed