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