libct/cg: rm "unsupported.go" files

These are not needed as these packages (libcontainer/cgroups,
libcontainer/cgroups/fs, and libcontainer/cgroups/systemd) can
not be built under non-linux anyway (for various reasons).

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin