Cleanup chrono includes

[tautschnig]

With Ubuntu 22.04 GitHub runner image 20221119.2 there appears to be some libclang difference so that include-what-you-use newly started suggesting bits/chrono instead of chrono (which is spurious); nevertheless, some of the uses of #include <chrono> indeed were unnecessary.

Fixes: #7383

• 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

Base: 78.35% // Head: 78.35% // No change to project coverage 👍

Coverage data is based on head (25c32cc) compared to base (53969bc).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7384   +/-   ##
========================================
  Coverage    78.35%   78.35%           
========================================
  Files         1645     1645           
  Lines       190208   190208           
========================================
  Hits        149040   149040           
  Misses       41168    41168           

Impacted Files	Coverage Δ
src/goto-checker/multi_path_symex_checker.cpp	95.65% <ø> (ø)
src/goto-checker/multi_path_symex_only_checker.cpp	100.00% <ø> (ø)
...-checker/single_loop_incremental_symex_checker.cpp	90.82% <ø> (ø)
src/goto-checker/single_path_symex_checker.h	100.00% <ø> (ø)
...rc/goto-checker/single_path_symex_only_checker.cpp	97.53% <ø> (ø)
src/goto-checker/single_path_symex_only_checker.h	100.00% <ø> (ø)
src/goto-checker/symex_coverage.cpp	96.35% <ø> (ø)
src/goto-symex/symex_target_equation.cpp	95.38% <ø> (ø)
src/solvers/prop/prop_conv_solver.cpp	87.50% <ø> (ø)

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