CHC encoder: fix function pointer removal

[kroening]

This fixes the removal of function pointers following the API change in 86c8181.

• 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

Base: 77.82% // Head: 77.82% // No change to project coverage 👍

Coverage data is based on head (2a08d59) compared to base (5e2ff66).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7183   +/-   ##
========================================
  Coverage    77.82%   77.82%           
========================================
  Files         1578     1578           
  Lines       182242   182242           
========================================
  Hits        141824   141824           
  Misses       40418    40418           

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