Remove `project_id` delegation from the `Ci::Pipeline` model

Delegating the `project_id` to `project` relation of the `Ci::Pipeline`
requires ActiveRecord to load the object from the database. Since we
already have the `project_id` foreign key on `Ci::Pipeline` we can just
use the value stored in that column.

app/models/ci/pipeline.rb

@@ -104,7 +104,6 @@ module Ci
 
     accepts_nested_attributes_for :variables, reject_if: :persisted?
 
-    delegate :id, to: :project, prefix: true
     delegate :full_path, to: :project, prefix: true
 
     validates :sha, presence: { unless: :importing? }