CONTRACTS: Replace return_value_visitor by has_subexpr

[tautschnig]

There is no need to create infrastructure when an existing utility can
be used instead.

• 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

Merging #6797 (2265264) into develop (b7c6692) will increase coverage by 0.06%.
The diff coverage is 89.40%.

@@             Coverage Diff             @@
##           develop    #6797      +/-   ##
===========================================
+ Coverage    76.90%   76.97%   +0.06%     
===========================================
  Files         1590     1593       +3     
  Lines       183969   184099     +130     
===========================================
+ Hits        141486   141713     +227     
+ Misses       42483    42386      -97     

Impacted Files	Coverage Δ
...rc/goto-instrument/contracts/memory_predicates.cpp	83.52% <ø> (-0.57%)	⬇️
src/goto-instrument/contracts/memory_predicates.h	100.00% <ø> (ø)
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	84.87% <76.74%> (+12.78%)	⬆️
src/solvers/smt2_incremental/object_tracking.cpp	97.87% <97.87%> (ø)
src/goto-instrument/contracts/contracts.cpp	93.84% <100.00%> (ø)
src/goto-programs/goto_function.cpp	100.00% <100.00%> (ø)
src/solvers/smt2_incremental/object_tracking.h	100.00% <100.00%> (ø)
...ncremental/smt2_incremental_decision_procedure.cpp	94.15% <100.00%> (+0.14%)	⬆️
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	100.00% <100.00%> (ø)
... and 8 more

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[SaswatPadhi]

Nice! Thanks