[AVR] Remove AVR-specific logic from libstd

It is not possible to compile libstd for AVR anyway.

Author: dylanmckay

src/libstd/env.rs

@@ -874,7 +874,6 @@ pub mod consts {
     /// - x86_64
     /// - arm
     /// - aarch64
-    /// - avr
     /// - mips
     /// - mips64
     /// - powerpc
@@ -987,11 +986,6 @@ mod arch {
     pub const ARCH: &str = "aarch64";
 }
 
-#[cfg(target_arch = "avr")]
-mod arch {
-    pub const ARCH: &'static str = "avr";
-}
-
 #[cfg(target_arch = "mips")]
 mod arch {
     pub const ARCH: &str = "mips";