Update tests and fix for parallel layout

Add "cell" tests to parallel diff row
Remove references to DiffTableCell
Update added table cell code to use appropriate "side"

changelogs/unreleased/jdb-refactor-parallel-diff-table-row.yml

+---
+title: Jdb/refactor parallel diff table row
+merge_request: 41606
+author:
+type: performance

qa/qa/page/merge_request/show.rb

@@ -53,10 +53,6 @@ module QA
           element :diffs_tab
         end
 
-        view 'app/assets/javascripts/diffs/components/diff_table_cell.vue' do
-          element :diff_comment
-        end
-
         view 'app/assets/javascripts/diffs/components/inline_diff_table_row.vue' do
           element :new_diff_line
         end