Remove the deny(warnings) attributes

It was suggested on the Rust Embedded channel to not set this in
released code.  The warnings are currently because of the embedded-hal
crate deprecating digital::v1, but stm32f4xx-hal still using it.

examples/serial.rs

@@ -4,7 +4,6 @@
 //! example with serial terminal emulator like `minicom`.
 
 #![deny(unsafe_code)]
-#![deny(warnings)]
 #![no_main]
 #![no_std]
 

examples/spi.rs

@@ -4,7 +4,6 @@
 //! debug output.
 
 #![deny(unsafe_code)]
-#![deny(warnings)]
 #![no_main]
 #![no_std]
 

src/main.rs

@@ -4,7 +4,6 @@
 //! ring does: cycle clock-wise, counter clock-wise, or follow the accelerometer.
 
 #![deny(unsafe_code)]
-#![deny(warnings)]
 #![no_main]
 #![no_std]