Suppress “Try Gutenberg” callout in WordPress 4.9.8

[greatislander]

Prerequisites

• Can you reproduce the behavior you wish to address with all other plugins deactivated?
• Are you running the latest version of Pressbooks and the latest version of WordPress?
• Are you experiencing this issue with one of our built-in root or book themes?
• Did you perform a cursory search to see if your bug or enhancement is already listed?

Description

WordPress 4.9.8 will introduce a call-to-action encouraging users to try Gutenberg. We need to hide this for the time being. See:

• https://core.trac.wordpress.org/ticket/41316
• https://github.com/senlin/classic-editor-addon/blob/master/classic-editor-addon.php#L57

Steps to Reproduce

1. Install WordPress 4.9.8-beta.2.

Expected behavior: I don't want to be prompted to install Gutenberg.

Actual behavior: I am prompted to install Gutenberg.

System Information

Pressbooks: 5.5.0-dev
WordPress: 4.9.8-beta.2

[greatislander]

Maybe not needed? https://core.trac.wordpress.org/ticket/44680

[greatislander]

Not needed because @connerbw already did this in 2f4abf7 🙃

[dac514]

Improved here: #1308