Remove c_enum_constant_type

[tautschnig]

This was put in place to succeed enum_constant_type (which was then deprecated), but neither of which are used for the C front-end has it's own (much more elaborate) way of doing enum_constant_type.

• 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

Base: 77.88% // Head: 77.88% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (8fe28ff) compared to base (ee0a06d).
Patch coverage: 90.54% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7153   +/-   ##
========================================
  Coverage    77.88%   77.88%           
========================================
  Files         1576     1576           
  Lines       181867   181863    -4     
========================================
  Hits        141645   141645           
+ Misses       40222    40218    -4     

Impacted Files	Coverage Δ
src/ansi-c/expr2c.cpp	67.13% <ø> (ø)
src/goto-programs/restrict_function_pointers.h	100.00% <ø> (ø)
src/goto-programs/validate_goto_model.h	100.00% <ø> (ø)
src/util/c_types.cpp	91.16% <ø> (+1.97%)	⬆️
src/util/c_types.h	100.00% <ø> (ø)
src/util/format_expr.cpp	86.89% <0.00%> (ø)
src/util/simplify_expr_class.h	90.47% <ø> (ø)
unit/goto-programs/goto_program_validate.cpp	100.00% <ø> (ø)
src/goto-programs/restrict_function_pointers.cpp	82.15% <50.00%> (ø)
src/ansi-c/c_typecheck_expr.cpp	74.19% <72.72%> (ø)
... and 24 more

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