N-10: remove unused constant and modifiers

[valera-grinenko-ai]

Summary

Addressed

• Remove unused INTEROP_BALANCE_CHANGE_VERSION constant from IAssetTrackerBase.sol (zero references in codebase)
• Remove unused onlyCallFromBootloaderOrInteropHandler and onlyCallFromInteropCenterOrNTV modifiers from SystemContractBase.sol (zero references), along with their now-unused imports
• Move _sendL1ToGatewayMigrationDataToL1 from AssetTrackerBase to L2AssetTracker (its only caller)
• Move _sendGatewayToL1MigrationDataToL1 from AssetTrackerBase to GWAssetTracker (its only caller)
• Clean up now-unused imports in AssetTrackerBase

Skipped

• SAVED_TOTAL_SUPPLY / isSaved field: actively used in L2AssetTracker.sol:119 — audit claim is incorrect
• Redundant imports (FinalizeDepositParams, IBridgehubBase, MigrationConfirmationData): claims don't hold for current code — these symbols are either directly used in contract bodies or no longer present
• receive() in BaseTokenHolder: used on ZK OS chains during L2BaseTokenZKOS.initL2() which sends ETH via Address.sendValue — audit only considered the Era path where balance is set via storage
• Chain ID check in _finalizeBridgeBurn: defense-in-depth; the block.chainid == _l1ChainId() guard makes the L1-only intent explicit at the call site even though _recordMigrationToSL is only implemented on L1

Test plan

• Verify forge build passes for both l1-contracts and system-contracts
• Confirm no references to removed symbols exist
• Confirm L2AssetTracker and GWAssetTracker migration flows work with relocated functions

🤖 Generated with Claude Code

[StanislavBreadless]

The following is missing:

The [SAVED_TOTAL_SUPPLY](https://github.com/matter-labs/era-contracts/blob/4271d0a37d64c43697781fa272d53e6b9c9f401c/l1-contracts/contracts/bridge/asset-tracker/IAssetTrackerBase.sol#L10) struct is defined within the base contract's interface while it is only used within the L2AssetTracker contract. Additionally, there seems to be no actual need for this struct since the [bool isSaved](https://github.com/matter-labs/era-contracts/blob/84e5c59386b6b324d6547c9ad551e43f9a22fc93/l1-contracts/contracts/bridge/asset-tracker/IAssetTrackerBase.sol#L11) field is never used, since [isAssetRegistered](https://github.com/matter-labs/era-contracts/blob/4271d0a37d64c43697781fa272d53e6b9c9f401c/l1-contracts/contracts/bridge/asset-tracker/IAssetTrackerBase.sol#L24) is used for all the checks instead.

And this one:

In _finalizeBridgeBurn, [_recordMigrationToSL](https://github.com/matter-labs/era-contracts/blob/4271d0a37d64c43697781fa272d53e6b9c9f401c/l1-contracts/contracts/core/chain-asset-handler/ChainAssetHandlerBase.sol#L254) is conditionally called only when block.chainId equals L1, but this check is redundant since _recordMigrationToSL is only implemented in L1ChainAssetHandler.