Remove unnecessary size_t -> unsigned cast

[tautschnig]

l1_index is (only) passed to a function that expects a size_t-typed
argument.

• 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 #6051 (e09de8a) into develop (ff5902e) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #6051      +/-   ##
===========================================
+ Coverage    74.25%   74.29%   +0.03%     
===========================================
  Files         1444     1444              
  Lines       157460   157460              
===========================================
+ Hits        116928   116982      +54     
+ Misses       40532    40478      -54     

Impacted Files	Coverage Δ
src/goto-symex/goto_symex_state.cpp	91.54% <100.00%> (ø)
src/util/c_types.cpp	90.05% <0.00%> (+0.55%)	⬆️
src/ansi-c/c_qualifiers.cpp	83.13% <0.00%> (+8.43%)	⬆️
src/goto-programs/link_goto_model.cpp	77.96% <0.00%> (+77.96%)	⬆️

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