DFCC: do not surface confusing warning

[tautschnig]

Users do not know that DFCC uses generate-function-bodies to ensure sound operation, and indeed the user had everything set up just right when no function needed to be generated.

Resolves: #8638

• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.39%. Comparing base (2e6200a) to head (f5b5575).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8694   +/-   ##
========================================
  Coverage    80.39%   80.39%           
========================================
  Files         1688     1688           
  Lines       207403   207403           
  Branches        73       73           
========================================
  Hits        166749   166749           
  Misses       40654    40654           

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