koordlet: cleanup cgroup path utils

[saintube]

Ⅰ. Describe what this PR does

1. Cleanup useless utility functions in the koordlet.
2. Move out the functions for cgroup parent generating for extensions.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Describe how to verify it

Ⅳ. Special notes for reviews

V. Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests
• All checks passed in make test

[codecov]

Codecov Report

Patch coverage: 90.56% and project coverage change: +0.14 🎉

Comparison is base (0048950) 66.51% compared to head (ca304ac) 66.65%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1120      +/-   ##
==========================================
+ Coverage   66.51%   66.65%   +0.14%     
==========================================
  Files         265      266       +1     
  Lines       29362    29210     -152     
==========================================
- Hits        19529    19470      -59     
+ Misses       8445     8353      -92     
+ Partials     1388     1387       -1     

Flag	Coverage Δ
unittests	66.65% <90.56%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/koordlet/util/stat.go	75.00% <ø> (+21.57%)	⬆️
pkg/koordlet/util/system/cgroup_resource.go	80.68% <ø> (ø)
pkg/koordlet/util/pod.go	33.33% <66.66%> (-26.67%)	⬇️
pkg/koordlet/util/container.go	69.49% <84.61%> (+8.48%)	⬆️
pkg/koordlet/util/cgroup_parent_utils.go	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[zwzhang0107]

/lgtm
/approve

[koordinator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zwzhang0107

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