fix: remove invalid param from wp_register_style() in gutenberg_enqueue_stored_styles()

[justlevine]

What?

This PR fixes an issue in gutenberg_enqueue_stored_styles() where wp_register_style() is called with an invalid $media arg value of true.

To resolve, the entire $media arg has been removed from the function call, in favor of the default $media = 'all'.

Why?

While this issue was surfaced by PHPStan (via #66693) it can be remediated independently as part of #66598

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

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Co-authored-by: justlevine <[email protected]>

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