Remove duplicate implementation of stash_variables

[tautschnig]

The implementation in acceleration_utilst has exactly the same code.

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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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
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[codecov]

Codecov Report

Merging #6725 (706f9d5) into develop (9053c5f) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           develop    #6725   +/-   ##
========================================
  Coverage    76.85%   76.85%           
========================================
  Files         1589     1589           
  Lines       183846   183835   -11     
========================================
  Hits        141289   141289           
+ Misses       42557    42546   -11     

Impacted Files	Coverage Δ
...o-instrument/accelerate/polynomial_accelerator.cpp	0.00% <0.00%> (ø)
...oto-instrument/accelerate/polynomial_accelerator.h	100.00% <ø> (ø)

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