chore: remove unnecessary type assertion

[jh-leong]

In this commit, I removed an unnecessary type assertion in the vOn.ts file.

[github-actions]

Size Report

Bundles

File	Size	Gzip	Brotli
runtime-dom.global.prod.js	86.5 kB	32.9 kB	29.7 kB
vue.global.prod.js	132 kB	49.6 kB	44.5 kB

Usages

Name	Size	Gzip	Brotli
createApp	47.9 kB	18.9 kB	17.2 kB
createSSRApp	51.2 kB	20.2 kB	18.4 kB
defineCustomElement	50.3 kB	19.7 kB	17.9 kB
overall	61.3 kB	23.7 kB	21.6 kB

[Alfred-Skyblue]

In #8978, the @typescript-eslint/eslint-plugin plugin was added for unified processing.

[jh-leong]

In #8978, the @typescript-eslint/eslint-plugin plugin was added for unified processing.

Got it! Using ESLint to avoid these issues is a more effective approach. I'll close this PR now.