Remove placeholder schema from helpers

Placeholder schemas were a transitionary state between Whitehall and other
publishing apps.

They are now redundant, and we are in the process of removing them.

Where they were used in specs, they have been swapped to the "generic" schema.

test/support/content_store_helpers.rb

@@ -24,7 +24,7 @@ def basic_taxon
     }
   end
 
-  def content_store_has_random_item(base_path:, schema: "placeholder", is_tagged_to_taxon: false, details: {})
+  def content_store_has_random_item(base_path:, schema: "generic", is_tagged_to_taxon: false, details: {})
     content_item = GovukSchemas::RandomExample.for_schema(frontend_schema: schema) do |item|
       taxons = is_tagged_to_taxon ? [basic_taxon] : []
       item.merge!(
@@ -47,7 +47,7 @@ def stub_content_store_has_item_tagged_to_taxon(base_path:, payload:)
     stub_content_store_has_item(base_path, content_item)
   end
 
-  def content_store_has_page(slug, schema: "placeholder", is_tagged_to_taxon: false)
+  def content_store_has_page(slug, schema: "generic", is_tagged_to_taxon: false)
     content_store_has_random_item(base_path: "/#{slug}", schema:, is_tagged_to_taxon:)
   end