Remove `mutable_borrow_reservation_conflict` allow

[jason-h-35]

warning: lint `mutable_borrow_reservation_conflict` has been removed: now allowed, see issue #59159 <https://github.com/rust-lang/rust/issues/59159> for more information
    --> src/session.rs:2681:21
     |
2681 |             #[allow(mutable_borrow_reservation_conflict)]
     |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(renamed_and_removed_lints)]` on by default

warning: `rx` (lib) generated 1 warning

This lint has been removed. See https://github.com/rust-lang/rust/issues/59159. This removes the allow for this now non-existent lint. 😃