Build Tooling: Remove WebpackRTLPlugin

[aduth]

Closes #15146

This pull request seeks to remove the WebpackRTLPlugin. The plugin is no longer necessary, as there is otherwise RTL CSS generated through the packages build script. The Webpack build configuration is only responsible for copying these built files:

gutenberg/webpack.config.js

Lines 85 to 107 in d08dcd8

	new CopyWebpackPlugin(
	gutenbergPackages.map( ( packageName ) => ( {
	from: `./packages/${ packageName }/build-style/*.css`,
	to: `./build/${ packageName }/`,
	flatten: true,
	transform: ( content ) => {
	if ( defaultConfig.mode === 'production' ) {
	return postcss( [
	require( 'cssnano' )( {
	preset: [ 'default', {
	discardComments: {
	removeAll: true,
	},
	} ],
	} ),
	] )
	.process( content, { from: 'src/app.css', to: 'dest/app.css' } )
	.then( ( result ) => result.css );
	}
	return content;
	},
	} ) )
	),

The plugin is also slated to become broken due to removed deprecations with Webpack 5.x , and would otherwise need to be updated.

Testing Instructions:

Verify there are no regressions in the CSS RTL output from the build.

cc @yoavf