Revert "specify workflow build in requirements"

This reverts commit 40bf52e2de6475a289c0c52944bd5b7a7c0a349f.

requirements/base.txt

@@ -1,5 +1,5 @@
 Django>=3.0,<3.1
 django-dotenv==1.4.1
-http://matthew-tbx.s3.amazonaws.com/wagtail/wagtail-workflow20200224-2.8a0.tar.gz
+wagtail>=2.8,<2.9
 wagtailfontawesome>=1.1.3,<1.2
 Pillow>=6.2.0,<6.3