Remove hardcoded field filter to allow custom discovery search fields

Description

Course discovery was unable to use SearchFilterGenerator for custom field filters because it only recognized `org`. In our case for example, we needed to autofilter with a select group of course ids.
This change is backwards compatible.

@nedbat

Assignee

Calen Pennington

Reporter

Open Source Pull Request Bot

Contributor Name

Kevin Luo

Repo

edx/edx-search

Customer

Epic Link

None

OSCM Assignee

None

Platform Map Area (Levels 1 & 2)

None

Platform Map Area (Levels 3 & 4)

None

Blended Hour Utilization Percentage

None

Priority

Unset
Configure