Value set: remove array-of-array special case

[tautschnig]

There should not be a need for get_reference_set to consider array-of-array typed expressions to have an unknown offset when others firmly have offset zero.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.36%. Comparing base (e81ad69) to head (4aa4388).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8653      +/-   ##
===========================================
- Coverage    80.36%   80.36%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       206981   206977       -4     
  Branches        73       73              
===========================================
- Hits        166337   166333       -4     
  Misses       40644    40644              

☔ 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.