Avoid creating labels when removing them

IssuableBaseService has been updated so that labels are not
created when push options to remove them are received.

app/services/issuable_base_service.rb

@@ -85,7 +85,7 @@ class IssuableBaseService < BaseService
     if params[:remove_label_ids]
       params[:remove_label_ids] = labels_service.filter_labels_ids_in_param(:remove_label_ids)
     elsif params[:remove_labels]
-      params[:remove_label_ids] = labels_service.find_or_create_by_titles(:remove_labels).map(&:id)
+      params[:remove_label_ids] = labels_service.find_or_create_by_titles(:remove_labels, find_only: true).map(&:id)
     end
 
     if params[:label_ids]

app/services/labels/available_labels_service.rb

@@ -9,7 +9,7 @@ module Labels
       @params = params
     end
 
-    def find_or_create_by_titles(key = :labels)
+    def find_or_create_by_titles(key = :labels, find_only: false)
       labels = params.delete(key)
 
       return [] unless labels
@@ -23,7 +23,7 @@ module Labels
           include_ancestor_groups: true,
           title: label_name.strip,
           available_labels: available_labels
-        ).execute
+        ).execute(find_only: find_only)
 
         label
       end.compact

app/services/labels/find_or_create_service.rb

@@ -9,9 +9,9 @@ module Labels
       @params = params.dup.with_indifferent_access
     end
 
-    def execute(skip_authorization: false)
+    def execute(skip_authorization: false, find_only: false)
       @skip_authorization = skip_authorization
-      find_or_create_label
+      find_or_create_label(find_only: find_only)
     end
 
     private
@@ -30,9 +30,11 @@ module Labels
     # Only creates the label if current_user can do so, if the label does not exist
     # and the user can not create the label, nil is returned
     # rubocop: disable CodeReuse/ActiveRecord
-    def find_or_create_label
+    def find_or_create_label(find_only: false)
       new_label = available_labels.find_by(title: title)
 
+      return new_label if find_only
+
       if new_label.nil? && (skip_authorization || Ability.allowed?(current_user, :admin_label, parent))
         create_params = params.except(:include_ancestor_groups)
         new_label = Labels::CreateService.new(create_params).execute(parent_type.to_sym => parent)