Keep symbols as nondet rather than using their symbol table values in SMT decision procedure.

[thomasspriggs]

This PR keeps symbols as nondet rather than using their symbol table values in SMT decision procedure.
This is needed because the value of a symbol is not intended to be defined until it has been expressed via expressions explicitly passed to the decision procedure.

This should help with this PR - #7858

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• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (64fe4d0) 79.07% compared to head (c3a910f) 79.08%.
Report is 7 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8104      +/-   ##
===========================================
+ Coverage    79.07%   79.08%   +0.01%     
===========================================
  Files         1698     1698              
  Lines       196464   196437      -27     
===========================================
+ Hits        155347   155349       +2     
+ Misses       41117    41088      -29     

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[tautschnig]

There now appear to be some includes that are no longer required.