cleanup: remove unnecessary 'allow' directives

I probably added these a while ago during some work and forgot to remove
them.

Author: BurntSushi

examples/graphemes-std.rs

@@ -16,8 +16,7 @@ fn main() -> Result<(), Box<dyn Error>> {
             .take(10)
             .last()
             .unwrap_or(line.len());
-        #[allow(deprecated)] // for Rust 1.28.0
-        stdout.write_all(line[..end].trim_right().as_bytes())?;
+        stdout.write_all(line[..end].trim_end().as_bytes())?;
         stdout.write_all(b"\n")?;
 
         line.clear();

src/ext_vec.rs

@@ -1,5 +1,3 @@
-#![allow(unused_imports)]
-
 use std::borrow::Cow;
 use std::error;
 use std::ffi::{OsStr, OsString};

src/lib.rs

@@ -367,7 +367,6 @@ Windows.
 */
 
 #![cfg_attr(not(feature = "std"), no_std)]
-#![allow(dead_code)]
 
 pub use crate::bstr::BStr;
 #[cfg(feature = "std")]