Fix: Double border on metabox panel

[jorgefilipecosta]

Fixes: #24612

This PR fixes a visual issue where two borders appear on the meta boxes panel

How has this been tested?

I installed yoast, and simple CSS plugin (both have a metabox).
I verified in the meta boxes panels there were no double borders.

[github-actions]

Size Change: -6 B (0%)

Total Size: 1.16 MB

Filename	Size	Change
build/edit-post/style-rtl.css	5.61 kB	-3 B (0%)
build/edit-post/style.css	5.61 kB	-3 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.67 kB	0 B
build/api-fetch/index.js	3.44 kB	0 B
build/autop/index.js	2.82 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	7.96 kB	0 B
build/block-directory/style-rtl.css	953 B	0 B
build/block-directory/style.css	952 B	0 B
build/block-editor/index.js	126 kB	0 B
build/block-editor/style-rtl.css	10.7 kB	0 B
build/block-editor/style.css	10.7 kB	0 B
build/block-library/editor-rtl.css	8.36 kB	0 B
build/block-library/editor.css	8.36 kB	0 B
build/block-library/index.js	132 kB	0 B
build/block-library/style-rtl.css	7.48 kB	0 B
build/block-library/style.css	7.49 kB	0 B
build/block-library/theme-rtl.css	729 B	0 B
build/block-library/theme.css	730 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	47.6 kB	0 B
build/components/index.js	200 kB	0 B
build/components/style-rtl.css	15.7 kB	0 B
build/components/style.css	15.7 kB	0 B
build/compose/index.js	9.68 kB	0 B
build/core-data/index.js	11.8 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.55 kB	0 B
build/date/index.js	5.38 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	4.47 kB	0 B
build/edit-navigation/index.js	11 kB	0 B
build/edit-navigation/style-rtl.css	1.12 kB	0 B
build/edit-navigation/style.css	1.12 kB	0 B
build/edit-post/index.js	304 kB	0 B
build/edit-site/index.js	17 kB	0 B
build/edit-site/style-rtl.css	3.06 kB	0 B
build/edit-site/style.css	3.06 kB	0 B
build/edit-widgets/index.js	11.8 kB	0 B
build/edit-widgets/style-rtl.css	2.45 kB	0 B
build/edit-widgets/style.css	2.45 kB	0 B
build/editor/editor-styles-rtl.css	537 B	0 B
build/editor/editor-styles.css	539 B	0 B
build/editor/index.js	45.3 kB	0 B
build/editor/style-rtl.css	3.8 kB	0 B
build/editor/style.css	3.79 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.71 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	621 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.52 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.11 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.33 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.41 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	13.9 kB	0 B
build/server-side-render/index.js	2.77 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.85 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

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