Clean up DriftClientBackend initialization

[jordy25519]

Removes the redundant getMultipleAccounts request using the data from the marketmaps instead

as a side affect it resolves issue for quicknode RPCs getting 413s

quicknode RPCs still getting 413s due to this request:

drift-rs/src/oraclemap.rs

Line 230 in 8aeed02

.get_multiple_accounts_with_config(&pubkeys, account_config)

can solve it by chunking the account requests but its hacky to maintain and may cause issue with other rpc providers