Remove unused goto_convertt::has_function_call

[tautschnig]

Its use (if ever) predates Git history. Removing to reduce the amount of dead code.

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (9c7bccc) 79.66% compared to head (5f53ea3) 79.66%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8201   +/-   ##
========================================
  Coverage    79.66%   79.66%           
========================================
  Files         1682     1682           
  Lines       195377   195370    -7     
========================================
  Hits        155641   155641           
+ Misses       39736    39729    -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.