docs: remove duplicate onDestroy description

[kidonng]

The same description is defined in both the docs and type definition:

svelte/documentation/docs/06-runtime/03-lifecycle-hooks.md

Lines 50 to 52 in a3e49b6

	Schedules a callback to run immediately before the component is unmounted.
	Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the only one that runs inside a server-side component.

svelte/packages/svelte/types/index.d.ts

Lines 361 to 368 in a3e49b6

	/**
	* Schedules a callback to run immediately before the component is unmounted.
	*
	* Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the
	* only one that runs inside a server-side component.
	*
	* */
	export function onDestroy(fn: () => any): void;

Since the docs also import the type definition, it resulted in the same content shown twice:

[changeset-bot]

⚠️ No Changeset found

Latest commit: 347f45e

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15274-svelte.vercel.app/