Cleanup build_sizeof_expr

[tautschnig]

This code is entirely specific to the C front-end. See each of the two commits for more details.

• Each commit message has a non-empty body, explaining why the change was made.
• 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 #6664 (673bd73) into develop (30d747b) will increase coverage by 0.00%.
The diff coverage is 85.94%.

@@           Coverage Diff            @@
##           develop    #6664   +/-   ##
========================================
  Coverage    76.76%   76.76%           
========================================
  Files         1582     1582           
  Lines       182357   182357           
========================================
+ Hits        139984   139988    +4     
+ Misses       42373    42369    -4     

Impacted Files	Coverage Δ
src/ansi-c/c_typecheck_base.h	100.00% <ø> (ø)
src/solvers/flattening/boolbv.h	62.50% <ø> (ø)
src/solvers/flattening/boolbv_byte_extract.cpp	83.11% <ø> (ø)
src/solvers/flattening/bv_utils.h	85.71% <ø> (ø)
src/util/piped_process.cpp	81.48% <ø> (ø)
src/util/pointer_offset_size.cpp	92.52% <ø> (+0.29%)	⬆️
src/solvers/flattening/boolbv_add_sub.cpp	48.33% <43.75%> (ø)
src/ansi-c/c_typecheck_expr.cpp	76.59% <83.87%> (ø)
src/ansi-c/expr2c.cpp	66.24% <93.10%> (+0.30%)	⬆️
src/solvers/flattening/bv_utils.cpp	79.61% <96.87%> (ø)
... and 31 more

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