Remove __CPROVER_malloc_size

[tautschnig]

Its use was deprecated since 2021-05-06.

• 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.
• 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 #6086 (9501bed) into develop (bfa4c2c) will decrease coverage by 0.04%.
The diff coverage is 64.96%.

@@             Coverage Diff             @@
##           develop    #6086      +/-   ##
===========================================
- Coverage    76.01%   75.96%   -0.05%     
===========================================
  Files         1527     1542      +15     
  Lines       164465   165005     +540     
===========================================
+ Hits        125013   125350     +337     
- Misses       39452    39655     +203     

Impacted Files	Coverage Δ
src/ansi-c/ansi_c_internal_additions.cpp	90.36% <ø> (ø)
src/cpp/cpp_internal_additions.cpp	90.09% <ø> (-0.10%)	⬇️
src/crangler/ctokenit.cpp	0.00% <0.00%> (ø)
src/crangler/ctokenit.h	0.00% <0.00%> (ø)
src/goto-programs/validate_goto_model.cpp	100.00% <ø> (ø)
src/goto-symex/frame.h	100.00% <ø> (ø)
src/goto-symex/goto_symex.h	92.30% <ø> (ø)
src/linking/remove_internal_symbols.cpp	100.00% <ø> (ø)
src/util/pointer_predicates.cpp	89.23% <ø> (+17.62%)	⬆️
src/util/pointer_predicates.h	60.00% <ø> (ø)
... and 26 more

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