Remove parameter redundancy for toKeyPair method

[acarnagey]

Used for wallet and crypto libraries.

index and hdPath are redundant.

Ethereum — m'/44’/${assumedCoinType}’/0’/0/${index}... vs .... m'/44’/60’/0’/0/0.

Should just use hdpath param, remove index.

[OR13]

This might effect a lot of stuff, and any static test vectors that were built from the old format will need to be aligned to the new method signature after the change is mage.