libct: Mount: rm {Pre,Post}mountCmds

[kolyshkin]

Those were added by commit 59c5c3a back in Apr 2015, but AFAICS were
never used and are obsoleted by more generic container hooks (initially
added by commit 05567f2 in Sep 2015).

[kolyshkin]

@rhatdan PTAL

[kolyshkin]

centos-stream-8 failure:

=== RUN   TestPodSkipDevicesUpdate
    systemd_test.go:104: open /sys/fs/cgroup/cpuset/system.slice/system-runc_test_podTestPodSkipDevicesUpdate.slice/cpuset.cpus: no such file or directory
--- FAIL: TestPodSkipDevicesUpdate (0.07s)

I have not seen it before. Filed #3351, CI job restarted.

[AkihiroSuda]

LGTM, but probably we should document this in the change log, although nobody is likely using libcontainer directly today.

[thaJeztah]

Also did a quick search, and only found two occurrences of this being used;

• In a MarkDown example describing a reproduction scenario for CVE-2021-30465 english translation
• In this project: github.com/bblfsh/bblfshd /cc @mcuadros @abeaumont @dennwc (FYI)

[thaJeztah]

LGTM

[thaJeztah]

I'll go ahead and merge, but feel free to comment if there are concerns in removing this