Remove exprt::make_bool

[kroening]

This method has been deprecated since June 2019, there are no in-tree uses
of this function and there is a simple substitute for external users.

• 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).
• n/a 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 #5550 (22d448b) into develop (bf62883) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5550   +/-   ##
========================================
  Coverage    68.52%   68.53%           
========================================
  Files         1187     1187           
  Lines        98265    98262    -3     
========================================
  Hits         67339    67339           
+ Misses       30926    30923    -3     

Flag	Coverage Δ
cproversmt2	42.97% <ø> (+<0.01%)	⬆️
regression	65.68% <ø> (+<0.01%)	⬆️
unit	32.26% <ø> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/util/expr.cpp	70.33% <ø> (+1.74%)	⬆️
src/util/expr.h	90.69% <ø> (ø)

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