remove ufmt re-export from avr-hal-generic

note: i've explicitly dropped the re-export of `ufmt` as it's not used
in this repository* and i don't see `ufmt` as an integral part of these
crates. consumers can (and should) just add it as their own dependency.
this is a breaking change for consumers which currently do something
like `use avr_hal_generic::ufmt`.

\* clarification: `ufmt` itself is being used in some places, but the
   re-export isn't.

avr-hal-generic/src/lib.rs

@@ -10,8 +10,6 @@ pub use avr_device;
 pub use nb;
 #[doc(hidden)]
 pub use paste;
-#[doc(hidden)]
-pub use ufmt;
 
 pub mod adc;
 pub mod clock;