spec: remove unused sequel gem

refs CNVS-34084

test plan: tests should pass

Change-Id: Idf0e3dde6a76eee2e84533be8eb687f5e1193f86
Reviewed-on: https://gerrit.instructure.com/98548
Tested-by: Jenkins
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>

Author: simonista

Gemfile.d/test.rb

@@ -34,7 +34,6 @@
 
   gem 'once-ler', '0.0.16'
 
-  gem 'sequel', '4.39.0', require: false
   # Keep this gem synced with docker-compose/seleniumff/Dockerfile
   gem 'selenium-webdriver', '2.53.4'
     gem 'childprocess', '0.5.9', require: false