kernel: remove ErrorReturnObj, C_NEW_STRING

[fingolfin]

We don't use them nor does any distributed package.

---

To be merged only after stable-4.16 has been created.

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.86%. Comparing base (e404ddd) to head (6b86f9c).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/error.g	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6415   +/-   ##
=======================================
  Coverage   78.86%   78.86%           
=======================================
  Files         685      685           
  Lines      293554   293539   -15     
  Branches     8672     8672           
=======================================
- Hits       231520   231514    -6     
+ Misses      60226    60218    -8     
+ Partials     1808     1807    -1     

☔ 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.