Enable the function-pointer fall-back assertion by default

[tautschnig]

Function pointer removal may be done implicitly as required by some
other operation that the user intended to do. If so, the user would not
know that they need to specify --pointer-check at that point. Therefore,
enable the ASSERT(false) in the else branch of function pointer
removal unconditionally.

• 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.
• 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 #7011 (86c8181) into develop (711cc67) will increase coverage by 0.00%.
The diff coverage is 97.05%.

@@           Coverage Diff            @@
##           develop    #7011   +/-   ##
========================================
  Coverage    77.85%   77.85%           
========================================
  Files         1569     1569           
  Lines       180670   180648   -22     
========================================
- Hits        140662   140646   -16     
+ Misses       40008    40002    -6     

Impacted Files	Coverage Δ
.../analyses/variable-sensitivity/write_stack_entry.h	100.00% <ø> (ø)
src/goto-cc/armcc_cmdline.cpp	22.22% <ø> (ø)
src/goto-cc/gcc_cmdline.cpp	79.23% <ø> (ø)
src/goto-cc/ms_cl_cmdline.cpp	0.00% <ø> (ø)
src/goto-instrument/value_set_fi_fp_removal.cpp	100.00% <ø> (ø)
src/goto-programs/restrict_function_pointers.cpp	80.67% <ø> (ø)
unit/solvers/smt2_incremental/object_tracking.cpp	100.00% <ø> (ø)
src/analyses/variable-sensitivity/write_stack.cpp	87.50% <94.11%> (-0.12%)	⬇️
jbmc/src/jdiff/jdiff_parse_options.cpp	66.31% <100.00%> (-0.36%)	⬇️
...nalyses/variable-sensitivity/write_stack_entry.cpp	100.00% <100.00%> (ø)
... and 8 more

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[tautschnig]

In follow-up work I will add an option to remove the assertion after previous instrumentation.