Fix line-number output in document-properties-*

[tautschnig]

The line_no variable was never used in producing the output, even though it was written to (and in parts also used in an expression without having an effect, which tripped up the compiler with newer standards versions).

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (4e4707c) 77.88% compared to head (2038010) 77.82%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7994      +/-   ##
===========================================
- Coverage    77.88%   77.82%   -0.06%     
===========================================
  Files         1701     1701              
  Lines       196427   196430       +3     
===========================================
- Hits        152981   152870     -111     
- Misses       43446    43560     +114     

Files	Coverage Δ
src/goto-instrument/document_properties.cpp	88.37% <100.00%> (-0.98%)	⬇️

... and 103 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.