[collectd/cpu] remove deprecated plugin

[atoulme]

Description:
Remove this deprecated plugin (deprecated as of 0.91.1).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 38.82%. Comparing base (5b6dcfe) to head (e5191e9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7097      +/-   ##
==========================================
- Coverage   38.86%   38.82%   -0.04%     
==========================================
  Files         348      347       -1     
  Lines       24246    24238       -8     
==========================================
- Hits         9423     9411      -12     
- Misses      14041    14042       +1     
- Partials      782      785       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.