Remove deprecated add_axioms_for_is_empty method

[tautschnig]

Fixes: #8019

• Each commit message has a non-empty body, explaining why the change was made.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

❌ Patch coverage is 0% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.42%. Comparing base (0170409) to head (84b72f5).

Files with missing lines	Patch %	Lines
...lvers/strings/string_constraint_generator_main.cpp	0.00%	11 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8915   +/-   ##
========================================
  Coverage    80.41%   80.42%           
========================================
  Files         1703     1703           
  Lines       188398   188398           
  Branches        73       73           
========================================
+ Hits        151496   151512   +16     
+ Misses       36902    36886   -16     

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