Remove use of `prelude_import`

[crawfxrd]

Probably means importing the prelude everywhere.

warning: the feature `prelude_import` is internal to the compiler or standard library
 --> src/main.rs:5:12
  |
5 | #![feature(prelude_import)]
  |            ^^^^^^^^^^^^^^
  |
  = note: using it is strongly discouraged
  = note: `#[warn(internal_features)]` on by default

warning: `system76_firmware_update` (bin "system76_firmware_update") generated 1 warning

[crawfxrd]

Resolved by #90