Symex guards: avoid unnecessary duplicate negation

[tautschnig]

We negated the right-hand side assigned to a guard symbol just so as to then negate the use of the guard symbol. Remove this duplicate negation.

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• 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).
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.39%. Comparing base (22d25d1) to head (573a8e6).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8041      +/-   ##
===========================================
- Coverage    80.39%   80.39%   -0.01%     
===========================================
  Files         1688     1688              
  Lines       207403   207402       -1     
  Branches        73       73              
===========================================
- Hits        166752   166749       -3     
- Misses       40651    40653       +2     

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