chore(shared): remove unused execa.ts module

[chenjiahan]

Summary

• Remove the unused execa.ts module, see https://github.com/web-infra-dev/rspress/pull/1773/files for related changes.
• Remove the execa dependency in the monorepo root, it's not used.

TODO

Considering that plugin-lasted-updated is a dependency of the Rspress core, perhaps plugin-lasted-updated can remove the execa to reduce dependencies.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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