removes merkle root comparison in erasure_mismatch (solana-labs#29447)

Merkle shreds within the same erasure batch have the same merkle root.
The root of the merkle tree is signed. So either the signatures match
or one fails sigverify, and the comparison of merkle roots is redundant.

ledger/src/shred/merkle.rs

@@ -336,12 +336,6 @@ impl ShredCode {
         Some(offset..offset + SIZE_OF_MERKLE_ROOT)
     }
 
-    pub(super) fn erasure_mismatch(&self, other: &ShredCode) -> bool {
-        shred_code::erasure_mismatch(self, other)
-            || self.merkle_root().ok() != other.merkle_root().ok()
-            || self.common_header.signature != other.common_header.signature
-    }
-
     fn from_recovered_shard(
         common_header: ShredCommonHeader,
         coding_header: CodingShredHeader,

ledger/src/shred/shred_code.rs

@@ -79,8 +79,15 @@ impl ShredCode {
     pub(super) fn erasure_mismatch(&self, other: &ShredCode) -> bool {
         match (self, other) {
             (Self::Legacy(shred), Self::Legacy(other)) => erasure_mismatch(shred, other),
-            (Self::Merkle(shred), Self::Merkle(other)) => shred.erasure_mismatch(other),
-            _ => true,
+            (Self::Legacy(_), Self::Merkle(_)) => true,
+            (Self::Merkle(_), Self::Legacy(_)) => true,
+            (Self::Merkle(shred), Self::Merkle(other)) => {
+                // Merkle shreds within the same erasure batch have the same
+                // merkle root. The root of the merkle tree is signed. So
+                // either the signatures match or one fails sigverify.
+                erasure_mismatch(shred, other)
+                    || shred.common_header().signature != other.common_header().signature
+            }
         }
     }
 }