Remove deprecated apis

[ceyonur]

Why this should be merged

Removes unnecessary, unused, deprecated code and APIs

How this works

This pull request involves significant changes to the codebase, primarily focusing on removing the multi-coin balance feature and simplifying the export transaction logic. Below are the most important changes grouped by theme:

Removal of Multi-Coin Balance Feature:

• core/genesis.go: Removed handling of MCBalance in the toBlock function.
• core/state/dump.go: Removed the IsMultiCoin field from the DumpAccount struct and related functions. [1] [2] [3]
• core/types/account.go: Removed the GenesisMultiCoinBalance type and MCBalance field from the GenesisAccount struct.
• core/types/gen_genesis_account.go: Removed the MCBalance field from the GenesisAccount struct in both MarshalJSON and UnmarshalJSON methods. [1] [2] [3] [4]

Simplification of Export Transaction Logic:

• plugin/evm/atomic/export_tx.go: Removed the assetID parameter and related logic from the NewExportTx and GetSpendableFunds functions, using ctx.AVAXAssetID directly. [1] [2] [3] [4] [5] [6]
• plugin/evm/client/client.go: Removed the ExportArgs struct and updated the Export function to use ExportAVAXArgs directly. [1] [2]

Test Updates:

• core/state/state_test.go: Removed isMultiCoin field from test cases in the TestDump and TestIterativeDump functions. [1] [2] [3] [4]
• plugin/evm/export_tx_test.go: Updated test cases to reflect the changes in the NewExportTx function. [1] [2]

Code Simplification:

• internal/ethapi/api.go: Simplified variable declarations in Inspect and Resend functions. [1] [2]

How this was tested

Need to bootstrap testnet/mainnet since it includes a change to genesis

Need to be documented?

Should remove docs containing these APIs

Need to update RELEASES.md?

Yes

[darioush]

couple nits, also let's try to figure out if it's in use before merging.