trie: make deletion of Trie more readable when match shortNode

[AshinGau]

This PR refactors the delete logic for shortNode in the MPT implementation to improve code readability and consistency with the insert function.

Motivation
Previously, the code used:

dirty, child, err := t.delete(n.Val, append(prefix, key[:len(n.Key)]...), key[len(n.Key):])

However, at this point in the code, matchlen is always equal to len(n.Key), so using key[:matchlen] and key[matchlen:] is both clearer and stylistically consistent with the insert function. This change makes the code easier to read and maintain, and reduces potential confusion for future contributors.