Remove mut in Rust sdk submit_tx

[pymongo]

Description

I want to use Arc share/clone Rust SDK EconiaClient, but submit_tx of EconiaClient is mutable can't used inside Arc by ref.

I notice that submit_tx mut sequence_number of account, but it is a Atomic do not require mut ref to modified it, So submit_tx do not need mutable and safely used inside Arc

Testing

cargo build

Checklist

• Did you update relevant documentation?
• Did you add tests to cover new code or a fixed issue?
• Did you update the changelog?
• Did you check off all checkboxes from the linked Linear task? (Ignore if
  you are not a member of Econia Labs)

If a task does not apply to your PR, strike it through and mark it complete:

This change is compatible with previous usage of submit_tx no need to write changelog

- [x] ~Did you update the changelog?~