Optimise query for #previous_environment_deployment

Rewrite to a much simpler query

- No need to search by name, unique index guarantees there can only be
one environment by that name for a project
- Also no need to have where claus for deployments.project_id. An
environment **must** belong to the same project_id anyway.

app/models/deployment.rb

@@ -251,11 +251,8 @@ class Deployment < ApplicationRecord
   end
 
   def previous_environment_deployment
-    project
-      .deployments
+    self.class.for_environment(environment_id)
       .success
-      .joins(:environment)
-      .where(environments: { name: environment.name })
       .where.not(id: self.id)
       .order(id: :desc)
       .take

changelogs/unreleased/optimise_previous_environment_deployment.yml

+---
+title: Optimise query for Deployment#previous_environment_deployment in LinkMergeRequestWorker
+merge_request: 57039
+author:
+type: performance