fix: remove default `q=50` Imgix parameter for Gatsby images

[angeloashmore]

Package

• [x] gatsby-source-prismic
• [x] gatsby-plugin-prismic-previews

Types of changes

• [ ] Chore (a non-breaking change which is related to package maintenance)
• [x] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

This PR removes the default q=50 Imgix URL parameter added to all images by gatsby-source-prismic and, by extension, gatsby-plugin-prismic-previews.

For more details on why this was changed, see: #498

Thanks to @jodiedoubleday for reporting this.

Checklist:

• [ ] My change requires an update to the official documentation.
• [x] All TSDoc comments are up-to-date and new ones have been added where necessary.
• [x] All new and existing tests are passing.

🐈