Tests: Remove Gutenberg plugin activation e2e test

[ockham]

Remove tests/e2e/specs/gutenberg-plugin.test.js from the 6.3 branch.

See #6396:

The purpose of tests/e2e/specs/gutenberg-plugin.test.js is to ensure that running the Gutenberg plugin (stable version) on a WordPress trunk install doesn't produce any fatals.

The test has been around since WP 6.2. It makes sense to have it present on older branches, as the Gutenberg plugin not only supports trunk, but also the current stable version of WordPress (i.e. currently 6.5), and one version below (6.4). However, it is not expected to work on any earlier versions beyond that; in practice, it has produced errors on some of those.
[...]
[#6396] cannot be cherry-picked to 6.3 and prior as the test was refactored (from Puppeteer to Playwright) after 6.3.

There's a Gutenberg counterpart PR (WordPress/gutenberg#60780) to bump the GB plugin's minimum required WP version to 6.4.

Trac ticket: https://core.trac.wordpress.org/ticket/60971

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[ockham]

Thank you @swissspidy!

Committed to Core's 6.3 branch in https://core.trac.wordpress.org/changeset/58047.