Restrict with_exprt to exactly three operands

[tautschnig]

We cannot build a C++ API for the previous >= 3, odd-number operand variant. Alas, we ended up with various places in the code base silently ignoring additional operands, which led to wrong verification results in Kani as recent changes in CBMC made increasing use of the value set (which is one of the places that silently ignored additional operands).

This is an alternative solution over #8668: #8668 does also fix the wrong verification results, but does not resolve original design problem. This PR now seeks to address both the bug and the design problem, and does so by largely just removing code.

Resolves: #7272

• 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.
• 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

Attention: Patch coverage is 92.38095% with 16 lines in your changes missing coverage. Please review.

Project coverage is 80.40%. Comparing base (48490fb) to head (fc5c5a0).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/goto-programs/graphml_witness.cpp	0.00%	12 Missing ⚠️
src/util/simplify_expr.cpp	92.30%	3 Missing ⚠️
...vers/smt2_incremental/encoding/struct_encoding.cpp	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8674   +/-   ##
========================================
  Coverage    80.39%   80.40%           
========================================
  Files         1688     1688           
  Lines       207387   207380    -7     
  Branches        73       73           
========================================
+ Hits        166731   166744   +13     
+ Misses       40656    40636   -20     

☔ 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.

[rod-chapman]

Test results for this branch running on mldsa-native/main are all good.

[rod-chapman]

Test results for this branch running on mlkem-native/main are all good.