Remove `has_many: sourced_pipelines` association in CI bridge/build job

What does this MR do and why?

Describe in detail what your merge request does and why.

The has_one :sourced_pipeline relationship was moved to Ci::Processable in !103555 (merged)

This MR removes the has_many :sourced_pipelines relationship from Ci::Bridge and Ci::Build

Background: The relation has_many :sourced_pipelines defined in Ci::Bridge and Ci::Build is not correct because a job can only trigger 1 downstream pipeline. This could cause confusion as well as data inconsistency if we ever end up having multiple downstream pipelines for a given job.

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Related to #218393 (closed)