Remove deprecated variants of find_symbols

[tautschnig]

It was deprecated in 2019 and had no in-tree users left.

• Each commit message has a non-empty body, explaining why the change was made.
• 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/
• n/a 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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[martin-cs]

LGTM

[codecov]

Codecov Report

Merging #5560 (0f6dd4a) into develop (ff1e81e) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5560   +/-   ##
========================================
  Coverage    69.01%   69.01%           
========================================
  Files         1187     1187           
  Lines        98257    98253    -4     
========================================
  Hits         67809    67809           
+ Misses       30448    30444    -4     

Flag	Coverage Δ
cproversmt2	43.09% <ø> (+<0.01%)	⬆️
regression	66.21% <ø> (+<0.01%)	⬆️
unit	32.26% <ø> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/util/find_symbols.cpp	84.70% <ø> (+3.80%)	⬆️
src/util/find_symbols.h	0.00% <ø> (ø)

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