Remove ID_width expression support from solvers

[tautschnig]

We do not seem to generate such an expression in the code base, and the
current implementation is rather dubious for it returns the multiple of
8 bits in the propositional encoding.

• 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 #6008 (ffd00f0) into develop (9ae905b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6008   +/-   ##
========================================
  Coverage    74.22%   74.23%           
========================================
  Files         1444     1444           
  Lines       157328   157309   -19     
========================================
- Hits        116783   116781    -2     
+ Misses       40545    40528   -17     

Impacted Files	Coverage Δ
src/goto-instrument/code_contracts.h	94.11% <ø> (ø)
src/solvers/flattening/boolbv.cpp	79.27% <ø> (+2.46%)	⬆️
src/solvers/smt2/smt2_conv.cpp	60.07% <ø> (+0.12%)	⬆️
src/goto-instrument/code_contracts.cpp	83.93% <100.00%> (ø)

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