Related: #18499
Effectively reverts #14420

This pull request seeks to remove the editor--prefixed legacy className compatibility. These were added in #14420 as a result of a migration of components from the @wordpress/editor to @wordpress/block-editor package. With the changed in #14420, class names were changed to adhere to the CSS naming conventions, while retaining the previously-existing editor- prefixed names. As of #18499, a backwards-compatibility statement has been added, clarifying that these classes are not treated as a public interface intended for backwards-compatibility. As such, they are proposed to be removed here.

This brings with it a few benefits:

• There is no longer any ambiguity on what is the intended canonical class name for these components. You can see in many of the updated files that there was inconsistency in how we referenced these classes, even within first-party code.
• Clarity in how new classes should be assigned to components, where previously it might have been expected that the editor- prefix be included for the sake of consistency, even when it was not strictly necessary for backwards-compatibility.
• Reduced bundle size. There appears to be ~4kb reclaimed from these changes in the block-editor bundle (331kb to 327kb minified, before gzip).

Implementation Notes:

My process was as follows:

To find the legacy classes, I performed a regular expression search on the files of the packages/block-editor folder, using the pattern (?<!block-)editor-. For each of these results, I manually verified and removed each compatibility class.

Once all the changes had been made, I compiled a complete set of folders where files had been modified, to use as the "Base" of our BEM naming convention to perform a search on the entire repository for lingering references to the legacy class names:

(?<!block-)editor-(compare|drop-zone|block-icon|block-inspector|block-contextual-toolbar|block-list|block-mover|block-navigation|block-settings-menu|block-styles|block-switcher|block-toolbar|block-types-list|color-palette|contrast-checker|default-block-appender|inner-blocks|inserter-with-shortcuts|inserter|link-control|media-placeholder|media-replace-flow|multi-selection-inspector|panel-color-settings|plain-text|rich-text|skip-to-selected-block|url-input|url-popover|warning|writing-flow)

This yielded many results where one of the following holds true:

• We were erroneously not using the correct, canonical class name (example)
• The code is old and unused (example)
• Class names were assigned to wrongly reuse styles from the block-editor package (example). Note: I did not update these issues here.

Testing Instructions:

Repeat testing instructions from #14420.