Remove --debug-level from cbmc and jbmc

[tautschnig]

This undocumented option no longer has any effect ever since
eb403a6 was merged.

• 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/
• 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

Merging #6518 (bb8c158) into develop (2e306c6) will increase coverage by 0.71%.
The diff coverage is 93.02%.

@@             Coverage Diff             @@
##           develop    #6518      +/-   ##
===========================================
+ Coverage    77.04%   77.76%   +0.71%     
===========================================
  Files         1594     1567      -27     
  Lines       185276   179624    -5652     
===========================================
- Hits        142748   139677    -3071     
+ Misses       42528    39947    -2581     

Impacted Files	Coverage Δ
jbmc/src/jbmc/jbmc_parse_options.cpp	76.09% <ø> (+0.11%)	⬆️
src/cbmc/cbmc_parse_options.cpp	79.16% <ø> (+0.14%)	⬆️
src/goto-instrument/cover_basic_blocks.h	100.00% <ø> (ø)
src/goto-instrument/source_lines.h	100.00% <ø> (ø)
src/goto-programs/builtin_functions.cpp	58.35% <0.00%> (ø)
src/goto-symex/symex_main.cpp	86.97% <ø> (-0.04%)	⬇️
src/jsil/jsil_convert.cpp	0.00% <0.00%> (ø)
src/solvers/smt2_incremental/object_tracking.h	100.00% <ø> (ø)
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
src/util/expr.h	96.90% <ø> (-0.27%)	⬇️
... and 59 more

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[peterschrammel]

I can't even remember that one :D 👻