Remove legacy CurrencyToVote in staking

[gpestana]

Remove the need for the CurrencyToVote converter type in the pallet-staking. This type converts coverts from BalanceOf<T> to a u64 "vote" type, and from a u128 (i.e. ExtendedBalance) back to BalanceOf<T>. In both conversion directions, the total issuance of the system must be provided.

The main reason for this conversion is that the current phragmen implementation does not correctly support types u128 as the main type. Thus, the conversion between balance (u128) and the supported "vote" u64 type.

Relying on the current issuance will be a problem with the staking parachain (let's assume that the staking runtime is not deployed in AH). In addition, it removing the need for this conversion will simplify and make it cheaper to run the stake-tracker and the associated list updates.

Solution should be getting rid of the need for CurrencyToVote adapter by ensuring all election algorithms can work with a Into<u128> score type.

[dharjeezy]

I noticed in the description for CurrencyToVote, the to_vote function returns a u64 instead of a u128. How will this suffice in this case since we care about using a u128? @gpestana