Remove the manual @jupyterlab/notebook-extension:panel settings fetch

[jtpio]

Description

In #319 we added the following logic to ensure the settings-defined toolbar items are correctly retrieved before creating a new notebook panel and adding it to the main area:

retrolab/packages/application-extension/src/index.ts

Lines 185 to 186 in 879e644

	await settingRegistry?.load('@jupyterlab/notebook-extension:panel');
	await Promise.resolve();

Expected behavior

Downstream distributions and extensions should ideally not have to explicit use that workaround.

Context

Noticed in #319

Allow extensions and users to customize easily toolbar items upstream PR: jupyterlab/jupyterlab#10469

The JupyterLab tests do something similar:

https://github.com/jupyterlab/jupyterlab/blob/50fa0047d63287005a2c3b1d8f5bfedfe56cde7e/packages/apputils/test/toolbar.spec.ts#L304-L308