remove deprecated split_string variant

[kroening]

This function has been deprecated since 14.11.2019, and the replacement is
easy to put in place.

• 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-io]

Codecov Report

Merging #5340 into develop will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #5340      +/-   ##
===========================================
- Coverage    68.16%   68.16%   -0.01%     
===========================================
  Files         1170     1170              
  Lines        96443    96441       -2     
===========================================
- Hits         65737    65735       -2     
  Misses       30706    30706              

Flag	Coverage Δ
#cproversmt2	42.47% <0.00%> (-0.01%)	⬇️
#regression	65.36% <100.00%> (-0.01%)	⬇️
#unit	31.76% <100.00%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/util/string_utils.h	100.00% <ø> (ø)
.../memory-analyzer/memory_analyzer_parse_options.cpp	70.37% <100.00%> (-0.54%)	⬇️
src/util/string_utils.cpp	91.93% <100.00%> (-0.13%)	⬇️

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