avr-hal-generic: prefer compilation error for unsupported targets

The use of "unimplemented!" delays error detection until runtime, but
the target architecture is known at compile time. By removing the
unimplemented function we prevent unsupported architectures at
compile-time.

Author: tones111

avr-hal-generic/src/delay.rs

@@ -57,11 +57,6 @@ fn busy_loop(mut c: u16) {
     }
 }
 
-#[cfg(not(target_arch = "avr"))]
-fn busy_loop(_c: u16) {
-    unimplemented!("Implementation is only available for avr targets!")
-}
-
 // Clock-Specific Delay Implementations ----------------------------------- {{{
 impl delay_v0::DelayUs<u16> for Delay<crate::clock::MHz24> {
     fn delay_us(&mut self, mut us: u16) {