remove_const_function_pointerst isn't a messaget

[tautschnig]

Use a messaget member instead as there is no is-a relationship between
remove_const_function_pointerst and messaget.

• 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 #5614 (82cfbd4) into develop (4144414) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5614   +/-   ##
========================================
  Coverage    69.32%   69.32%           
========================================
  Files         1241     1241           
  Lines       100432   100432           
========================================
  Hits         69625    69625           
  Misses       30807    30807           

Flag	Coverage Δ
cproversmt2	43.08% <66.66%> (ø)
regression	66.22% <100.00%> (ø)
unit	32.27% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...c/goto-programs/remove_const_function_pointers.cpp	81.21% <100.00%> (ø)
src/goto-programs/remove_const_function_pointers.h	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4144414...82cfbd4. Read the comment docs.