Cleanup and use propt::new_variables

[tautschnig]

Several places still locally had a for loop where each step performed a
call to propt::new_variable, which is all taken care of by
propt::new_variables(width). Also override this method in cnft to avoid
repeated calls to set_no_variables.

• 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.
• 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 #6053 (5283612) into develop (ff5902e) will increase coverage by 0.02%.
The diff coverage is 76.92%.

@@             Coverage Diff             @@
##           develop    #6053      +/-   ##
===========================================
+ Coverage    74.25%   74.28%   +0.02%     
===========================================
  Files         1444     1444              
  Lines       157460   157436      -24     
===========================================
+ Hits        116928   116951      +23     
+ Misses       40532    40485      -47     

Impacted Files	Coverage Δ
src/solvers/flattening/boolbv.cpp	80.00% <0.00%> (+0.47%)	⬆️
src/solvers/flattening/boolbv_case.cpp	0.00% <0.00%> (ø)
src/solvers/flattening/boolbv_cond.cpp	0.00% <0.00%> (ø)
src/solvers/prop/prop.cpp	42.85% <0.00%> (-42.86%)	⬇️
src/solvers/prop/prop.h	84.37% <ø> (ø)
src/solvers/sat/cnf.h	83.33% <ø> (ø)
src/solvers/flattening/boolbv_byte_extract.cpp	69.62% <100.00%> (-0.38%)	⬇️
src/solvers/flattening/boolbv_index.cpp	78.94% <100.00%> (-1.20%)	⬇️
src/solvers/flattening/bv_pointers.cpp	84.61% <100.00%> (-0.33%)	⬇️
src/solvers/flattening/bv_utils.cpp	77.33% <100.00%> (-0.15%)	⬇️
... and 4 more

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