Remove deprecated add_axioms_for_concat function

[tautschnig]

Fixes: #8013

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• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.41%. Comparing base (0170409) to head (889a3fc).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8913   +/-   ##
========================================
  Coverage    80.41%   80.41%           
========================================
  Files         1703     1703           
  Lines       188398   188411   +13     
  Branches        73       73           
========================================
+ Hits        151496   151515   +19     
+ Misses       36902    36896    -6     

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