test: move http header related tests from io tests to spec tests

Signed-off-by: Taimoor Zaeem <taimoorzaeem@gmail.com>

test/io/fixtures/schema.sql

@@ -259,13 +259,6 @@ create or replace function notify_pgrst() returns void as $$
   notify pgrst;
 $$ language sql;
 
-
-create or replace function custom_vary_hdr() returns void as $$
-  begin
-    perform set_config('response.headers', '[{"Vary": "X-Test-Accept"}]', false);
-  end
-$$ language plpgsql;
-
 create or replace function get_work_mem() returns text as $$
   select current_setting('work_mem', true);
 $$ language sql;