[backend] fix rels batch loading (#6635)

[SouadHadjiat]

Proposed changes

• create batch loader outside of loadThroughDenormalized, otherwise there was no batch loading, we were loading rels for each element one by one.
• remove unused graphql queries : countries on IPs and reports on stix core objects (they were not used and were time consuming)

Related issues

• Slow list entities graphQL queries with refs loading #6635

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.70%. Comparing base (d9694b1) to head (426d5e7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6636   +/-   ##
=======================================
  Coverage   67.69%   67.70%           
=======================================
  Files         532      532           
  Lines       65144    65144           
  Branches     5470     5473    +3     
=======================================
+ Hits        44101    44104    +3     
+ Misses      21043    21040    -3     

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