Fix showing wrong UUID, remove unused library

[UntriexTv]

In case of invalid UUID agent showed:

WARN  keylime_agent::config > Misformatted UUID: invalid-uuid-0909
INFO  keylime_agent::config > Using generated UUID: invalid-uuid-0909

and warning while compiling:

warning: unused import: `std::io::prelude`
    --> keylime/src/tpm.rs:1816:9
     |
1816 |     use std::io::prelude::*;
     |         ^^^^^^^^^^^^^^^^
     |
     = note: `#[warn(unused_imports)]` on by default