Remove deprecated string abstraction methods

[tautschnig]

Parts of the string_abstractiont API were deprecated in 2020 and can
safely be removed.

• 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/
• 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.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6673 (3f9f055) into develop (b26ad8b) will increase coverage by 0.05%.
The diff coverage is 99.55%.

@@             Coverage Diff             @@
##           develop    #6673      +/-   ##
===========================================
+ Coverage    76.76%   76.81%   +0.05%     
===========================================
  Files         1582     1582              
  Lines       182360   182762     +402     
===========================================
+ Hits        139992   140397     +405     
+ Misses       42368    42365       -3     

Impacted Files	Coverage Δ
src/goto-programs/string_abstraction.h	100.00% <ø> (ø)
src/goto-programs/string_abstraction.cpp	92.46% <88.23%> (+0.33%)	⬆️
...solvers/smt2_incremental/smt_bit_vector_theory.cpp	100.00% <100.00%> (ø)
...solvers/smt2_incremental/smt_bit_vector_theory.cpp	100.00% <100.00%> (ø)

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