ci(docker): use existing application setting

The application already has a PREREVIEW_ORCID_SANDBOX setting; the value in the YAML file was incorrect as it needs to
be a string rather than a boolean, so this removes it.

I'm not sure if this is the cause of the dev environment failing; I'd be surprised, but without feedback it's impossible
to tell.

Refs #390

deploy/docker-compose-dev.yml

@@ -19,7 +19,6 @@ services:
       PREREVIEW_DB_PORT: 5432
       PREREVIEW_DB_USER: prereview
       PREREVIEW_DB_NAME: prereview
-      PREREVIEW_ORCID_SANDBOX: true
 
   prereview-db:
     image: postgres:12-alpine