[coding] Remove redundant BMT proof verifications

[clabby]

Overview

While sampling #1680, I noticed that the reed solomon scheme redundantly verifies all shards against the commitment:

monorepo/coding/src/reed_solomon/mod.rs

Lines 279 to 282 in 2df7d13

	// Verify Merkle proof
	if !chunk.verify(chunk.index, root) {
	return Err(Error::InvalidProof);
	}

The Scheme::decode interface mandates the use of checked shards, so these additional verifications are not necessary. Over time, they seem to add up to quite a bit of work (7.1% total samples in a 1000 block sync of alto):