Remove vue_issues_list feature flag stubs

ee/spec/controllers/ee/groups_controller_spec.rb

ee/spec/controllers/projects/issues_controller_spec.rb

...issues/filtered_search/filter_issues_by_iteration_spec.rb

...ltered_search/filter_issues_by_multiple_assignees_spec.rb

...eatures/issues/filtered_search/filter_issues_epic_spec.rb

...tures/issues/filtered_search/filter_issues_weight_spec.rb

ee/spec/features/issues/user_sees_empty_state_spec.rb

ee/spec/features/issues/user_views_issues_spec.rb

spec/controllers/groups_controller_spec.rb

spec/controllers/projects/issues_controller_spec.rb

spec/features/groups/empty_states_spec.rb

spec/features/groups/issues_spec.rb

...roups/user_sees_users_dropdowns_in_issuables_list_spec.rb

spec/features/issuables/issuable_list_spec.rb

spec/features/issue_rebalancing_spec.rb

...features/issues/filtered_search/dropdown_assignee_spec.rb

spec/features/issues/filtered_search/dropdown_author_spec.rb

spec/features/issues/filtered_search/dropdown_base_spec.rb

spec/features/issues/filtered_search/dropdown_emoji_spec.rb

spec/features/issues/filtered_search/dropdown_hint_spec.rb