fix various warnings

[joehillen]

warning: unused `#[macro_use]` import
--> src/main.rs:3:1
  |
3 | #[ macro_use ]
  | ^^^^^^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

warning: unused `#[macro_use]` import
--> src/main.rs:6:1
  |
6 | #[ macro_use ]
  | ^^^^^^^^^^^^^^

warning: type alias is never used: `HashLists`
  --> src/types.rs:13:1
  |
13 | / pub type HashLists =
14 | |  HashMap <Hash, Vec <PathBuf>>;
  | |_______________________________^
  |
  = note: #[warn(dead_code)] on by default

warning: type alias is never used: `CompareFileMetadataLists`
  --> src/types.rs:22:1
  |
22 | / pub type CompareFileMetadataLists =
23 | |  HashMap <CompareFileMetadata, Vec <PathBuf>>;
  | |______________________________________________^

warning: variable does not need to be mutable
  --> src/commands/dedupe_command.rs:627:6
    |
627 |  let mut file_database =
    |      ^^^^^^^^^^^^^^^^^
    |
    = note: #[warn(unused_mut)] on by default

[jamespharaoh]

thanks