CONTRACTS: Drop function pointer contract support

[remi-delmas-3000]

Dropping support for function pointer contract clauses in the old contract system (now only supported with DFCC).
This feature only makes sense when combined with the ability to use C functions to specify assignable sets, which only
exists in DFCC.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Base: 78.28% // Head: 78.28% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (9fd95ed) compared to base (a295651).
Patch coverage: 82.45% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7328      +/-   ##
===========================================
- Coverage    78.28%   78.28%   -0.01%     
===========================================
  Files         1642     1642              
  Lines       190004   189992      -12     
===========================================
- Hits        148744   148734      -10     
+ Misses       41260    41258       -2     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
src/goto-programs/graphml_witness.cpp	52.94% <0.00%> (-0.15%)	⬇️
src/goto-programs/interpreter.cpp	52.38% <0.00%> (+0.17%)	⬆️
src/util/expr_initializer.cpp	85.10% <80.00%> (+1.77%)	⬆️
jbmc/src/java_bytecode/expr2java.cpp	86.25% <93.75%> (ø)
src/ansi-c/c_typecheck_initializer.cpp	76.57% <100.00%> (+0.04%)	⬆️
src/ansi-c/expr2c.cpp	67.33% <100.00%> (-0.02%)	⬇️
src/goto-instrument/contracts/contracts.cpp	94.47% <100.00%> (-0.56%)	⬇️
src/goto-instrument/dump_c.cpp	79.97% <100.00%> (-0.03%)	⬇️
src/memory-analyzer/analyze_symbol.cpp	74.09% <100.00%> (+0.06%)	⬆️
... and 4 more

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