Fix: Update clippy lints for Rust 1.54

[alamb]

Rationale:

It's that time again! Rust released 1.54 today which causes some additional clippy errors:

If you run clippy on master after running rustup update you get erros such as the following

error: this expression borrows a reference (`&datafusion::logical_plan::JoinType`) that is immediately dereferenced by the compiler
   --> ballista/rust/core/src/serde/physical_plan/mod.rs:108:21
    |
108 |                     &join_type,
    |                     ^^^^^^^^^^ help: change this to: `join_type`
    |
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

error: aborting due to previous error

error: this expression borrows a reference (`&ballista_core::config::BallistaConfig`) that is immediately dereferenced by the compiler
   --> ballista/rust/client/src/context.rs:136:13
    |
136 |             &guard.config(),
    |             ^^^^^^^^^^^^^^^ help: change this to: `guard.config()`
    |
    = note: `-D clippy::needless-borrow` implied by `-D warnings`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_borrow

...

Changes:

1. Fix clippy errors by doing its suggestions

[Dandandan]

Thanks @alamb for fixing it so fast

[alamb]

👍

[alamb]

LOL here is a sister PR for arrow-rs apache/arrow-rs#631