Remove unused make_free_bv_expr

[tautschnig]

This was previously used for encoding waitfor, but this is no longer
supported in this code base. With the removal of make_free_bv_expr,
make_bv_expr became unnecessary, and thereby expressions with id
ID_bv_literals can no longer be generated.

• 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 #6860 (ab7e173) into develop (5d74a03) will increase coverage by 0.00%.
The diff coverage is 74.45%.

@@           Coverage Diff            @@
##           develop    #6860   +/-   ##
========================================
  Coverage    77.78%   77.79%           
========================================
  Files         1567     1567           
  Lines       179701   179750   +49     
========================================
+ Hits        139789   139844   +55     
+ Misses       39912    39906    -6     

Impacted Files	Coverage Δ
src/analyses/interval_analysis.cpp	0.00% <0.00%> (ø)
src/ansi-c/expr2c.cpp	67.63% <ø> (+0.03%)	⬆️
src/goto-instrument/accelerate/accelerate.cpp	32.28% <0.00%> (+0.33%)	⬆️
.../goto-instrument/accelerate/acceleration_utils.cpp	2.23% <0.00%> (+<0.01%)	⬆️
src/goto-instrument/accelerate/trace_automaton.cpp	0.00% <ø> (ø)
src/goto-instrument/wmm/data_dp.cpp	75.71% <ø> (ø)
src/goto-programs/goto_program.h	90.70% <ø> (ø)
src/jsil/jsil_entry_point.cpp	0.00% <0.00%> (ø)
src/solvers/flattening/arrays.cpp	81.45% <ø> (ø)
src/solvers/flattening/boolbv.cpp	86.36% <ø> (+6.02%)	⬆️
... and 44 more

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