Remove local declaration of void_t

[tautschnig]

With the move to C++ 17 we can use std::void_t instead.

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• 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).
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a8de0b7) 79.10% compared to head (ab81bbd) 79.09%.
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8031      +/-   ##
===========================================
- Coverage    79.10%   79.09%   -0.01%     
===========================================
  Files         1699     1699              
  Lines       196508   196508              
===========================================
- Hits        155443   155430      -13     
- Misses       41065    41078      +13     

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[thomasspriggs]

@tautschnig Hope you don't mind me merging this. I want to raise this PR based on top of it - #8035