remove dead boolbvt::literals(...)

[kroening]

This method has no users, and can be removed.

Deprecation appears unnecessary given that external users of the API have
much better alternatives to achieve the same.

• X ] 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/
• n/a 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 #5810 (37762a1) into develop (5fd8562) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #5810      +/-   ##
===========================================
+ Coverage    69.72%   69.75%   +0.02%     
===========================================
  Files         1242     1242              
  Lines       100907   100873      -34     
===========================================
+ Hits         70361    70362       +1     
+ Misses       30546    30511      -35     

Flag	Coverage Δ
cproversmt2	43.40% <ø> (+0.02%)	⬆️
regression	66.71% <ø> (+0.02%)	⬆️
unit	32.30% <ø> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/solvers/flattening/boolbv.cpp	76.49% <ø> (+6.91%)	⬆️
src/solvers/flattening/boolbv.h	78.57% <ø> (ø)
src/solvers/prop/prop_conv_solver.cpp	87.82% <ø> (+1.92%)	⬆️
src/solvers/prop/prop_conv_solver.h	37.50% <ø> (ø)

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