nit(mpt): Remove unnecessary collapse_if_possible call

[hashcashier]

This aim of this PR is to remove a very subtle yet unused unblind call to prevent access to unused trie nodes. Such additional access, while having minor performance impact, prevents the delegation of execution witness generation to external clients that retrieve the minimum required trie nodes, such as zeth.

The PR modifies collapse_if_possible to avoid attempting to collapse the blinded child of an extension node. The reason is that because we assume the trie is correctly constructed prior to any execution, there is no need to unblind the child of the extension (which must be a branch) to attempt further collapse, because the execution did not unblind this child already and therefore did not perform any reads/writes to it.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.4%. Comparing base (2e943f4) to head (c89ece3).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/mpt/src/node.rs	0.0%	3 Missing ⚠️

Additional details and impacted files

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.