How do I removed accordions in the advanced search
Unchecking the box in the image below for each field in your custom data set ensures it is not searchable. If no fields within the custom data set are searchable, the "accordion" is removed from the advanced search. If just one is searchable, then the accordion appears. 

Just confirmed this with 5.10.3. 

As for your second question, if you want to rename "Custom Fields" to something else, you can do that using CiviCRM's word replacement tool. Go to Administer > Customize Data &amp; Screens > Word Replacement.