Remove GUTENBERG_PHASE configuration from webpack

[talldanwp]

The Gutenberg repo has been using a GUTENBERG_PHASE environment variable for a while to feature flag particular features (mostly experimental blocks).

When the packages from Gutenberg are compiled by webpack in core, the idea is that this feature flagged code is removed from the build via webpack's dead code elimination feature.

The variable was set to an integer representing the phases of Gutenberg (core set to 1 and gutenberg set to 2), so that when the project moves to a new phase the value is incremented. That didn't really turn out to be a useful approach though.

Recently in the Gutenberg project this variable was renamed to IS_GUTENBERG_PLUGIN and changed to a boolean (​https://github.com/WordPress/gutenberg/pull/38202).

It should now be possible to remove this configuration from WordPress core and rely on the falseyness of it being undefined.

Here are the details of what needs to be removed:
​https://github.com/WordPress/wordpress-develop/search?q=GUTENBERG_PHASE

Removes the GUTENBERG_PHASE configuration that was previously used to remove feature flagged gutenberg code. The way this works has changed in the Gutenberg codebase (see ​https://github.com/WordPress/gutenberg/pull/38202). The variable is now called IS_GUTENBERG_PLUGIN and is a boolean value.

It should be fine to remove this configuration as setting it to false would have the same result in it not being present, given undefined is also a falsey value.

Trac ticket: 55017

I'm not sure how to fix the Test NPM failure on CI, if anyone has advice that would be very much appreciated 😄

Running npm run build:dev should generate an updated script-loader-packages.php file.

Thanks, that's now updated 👍

I didn’t notice this PR before. We replaced this constant with the new one used in Gutenberg. See ​https://github.com/WordPress/wordpress-develop/commit/bab610091d59f6dd21c7db84ba3e1ca0bab6a211. Should we skip it altogether in WordPress core insteas?

@gziolo I think it's fine either way. Easiest option is for me to close this PR now 😄

Thanks for removing the old configuration. 🙇

[talldanwp]

As mentioned in the previous comments, this should be resolved, so I'm closing the ticket.