Prevent preloading unused relations in security report findings type

What does this MR do and why?

This change adds a test to ensure that an N+1 query is not present when loading merge requests in the pipelineSecurityReportFindingType and it removes preloading unused relations.

#387238 (closed)

Before:

       Expected a maximum of 16 queries, got 21:

       Query Diff:
       -----------
       SELECT "ci_pipelines".* FROM "ci_pipelines"...
       -- (expected: 0, got: 1)
          WHERE "ci_pipelines"."id" IN (575, 577, 578, 579, 576)

       SELECT "projects".* FROM "projects"...
       -- (expected: 0, got: 1)
          WHERE "projects"."id" = 1555

       SELECT "users".* FROM "users"...
       -- (expected: 0, got: 1)
          WHERE "users"."id" IN (2161, 2167, 2170, 2173, 2164)
       -- (expected: 0, got: 1)
          WHERE "users"."id" = 2150 LIMIT 1

       SELECT "merge_requests".* FROM "merge_requests"...
       -- (expected: 0, got: 1)
          WHERE "merge_requests"."id" IN (419, 421, 422, 423, 420)
     # ./ee/spec/graphql/types/pipeline_security_report_finding_type_spec.rb:359:in `block (4 levels) in <top (required)>'

After:

       Expected a maximum of 16 queries, got 18:

       Query Diff:
       -----------
       SELECT "merge_requests".* FROM "merge_requests"...
       -- (expected: 0, got: 1)
          WHERE "merge_requests"."id" IN (112, 110, 109, 108, 111)

       SELECT "users".* FROM "users"...
       -- (expected: 0, got: 1)
          WHERE "users"."id" IN (573, 577, 581, 585, 589)
     # ./ee/spec/graphql/types/pipeline_security_report_finding_type_spec.rb:365:in `block (4 levels) in <top (required)>'

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