Fix delete_all on association

Previously, having the `where` turned this into a relation
so `ActiveRecord::Relation#delete_all` is called.

In Rails 6, `ActiveRecord::Associations::CollectionProxy#delete_all`
is called causing an error in the query.

app/services/projects/detect_repository_languages_service.rb

@@ -12,7 +12,7 @@ module Projects
       matching_programming_languages = ensure_programming_languages(detection)
 
       RepositoryLanguage.transaction do
-        project.repository_languages.where(programming_language_id: detection.deletions).delete_all
+        RepositoryLanguage.where(project_id: project.id, programming_language_id: detection.deletions).delete_all
 
         detection.updates.each do |update|
           RepositoryLanguage

spec/models/merge_request_spec.rb

@@ -302,7 +302,11 @@ describe MergeRequest do
 
       it 'returns empty requests' do
         latest_merge_request_diff = merge_request.merge_request_diffs.create
-        latest_merge_request_diff.merge_request_diff_commits.where(sha: 'b83d6e391c22777fca1ed3012fce84f633d7fed0').delete_all
+
+        MergeRequestDiffCommit.where(
+          merge_request_diff_id: latest_merge_request_diff,
+          sha: 'b83d6e391c22777fca1ed3012fce84f633d7fed0'
+        ).delete_all
 
         expect(by_commit_sha).to be_empty
       end