Remove supportSlidingSync boolean.

[bmarty]

Note that this field was not used (read) anymore, so this is just cleaning up the code.

Context: #3605.

[github-actions]

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.67%. Comparing base (39d7084) to head (0018cce).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3609      +/-   ##
===========================================
- Coverage    82.68%   82.67%   -0.01%     
===========================================
  Files         1731     1730       -1     
  Lines        41008    40993      -15     
  Branches      4973     4971       -2     
===========================================
- Hits         33906    33892      -14     
  Misses        5342     5342              
+ Partials      1760     1759       -1     

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