Remove unused option "simplify-if"

[tautschnig]

Going back as far as version 2.9 of CBMC this option does not seem to be
read anywhere, it was just (unnecessarily) being set in the
front-end(s).

The command-line option "no-simplify-if" is left in place so as not to
break any scripts. A major version upgrade should take care of cleaning
this up as well.

• 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

Merging #6919 (4790e60) into develop (e42a46d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6919   +/-   ##
========================================
  Coverage    77.78%   77.79%           
========================================
  Files         1568     1568           
  Lines       180269   180295   +26     
========================================
+ Hits        140230   140258   +28     
+ Misses       40039    40037    -2     

Impacted Files	Coverage Δ
jbmc/src/jbmc/jbmc_parse_options.cpp	76.41% <ø> (+0.06%)	⬆️
src/cbmc/cbmc_parse_options.cpp	80.10% <ø> (+0.10%)	⬆️
src/util/config.cpp	57.85% <100.00%> (ø)
...t/solvers/smt2_incremental/convert_expr_to_smt.cpp	99.66% <0.00%> (+0.01%)	⬆️
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	87.80% <0.00%> (+0.10%)	⬆️

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