Remove Github URL when submitting work to Custom Issue

**User Story:**
As a contributo
I dont want to be asked for a github issue URL on custom issues
So that it doesnt confuse me and stop from submitting work

**Acceptance Criteria**
GIVEN I am viewign a bounty that was created from a custom issue
WHEN I am submitting work
THEN i shouldnt see the Github Issue URL on the submit work form... Should only show for Github issues

**Product & Design Links:**

Remove the field shown and shift everything up

Start by locating the submit-work form and the code that distinguishes custom issues from GitHub issues. Reproduce submission for a bounty created from a custom issue, then verify that the GitHub Issue URL field is absent there while remaining visible for GitHub issues.

Written by the indexing model from the issue text.