chore: Remove unused code across multiple modules

[Copilot]

Changes

Removed 287 lines of dead code across Core, Network, State, Trie, Consensus, Serialization, and chain-specific modules:

• Deleted unused helper classes: HashHelpers, InsertionBehavior, PeerEqualityComparer
• Removed orphaned XDC infrastructure: IXdcSealer, QuorumCertificateException, XdcDbNames
• Deleted unused SSZ container serialization utilities
• Cleaned up unused extension methods in BlockHeaderExtensions and PatriciaTree
• Removed dead fields and using directives across affected modules

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a change that causes existing functionality not to work as expected)
• Optimization
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

Documentation

Requires documentation update

• Yes
• No

Requires explanation in Release Notes

• Yes
• No

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