Remove unnecessary feature declarations

This CL removes `default_alloc_error_handler` annotations as this
feature has been stabilized in Rust 1.68.0.

Bug: 267698452
Test: m rust
Change-Id: I9c0c099a522a1c9cc3acf92d40a6fd6b8e14277f
diff --git a/pvmfw/src/main.rs b/pvmfw/src/main.rs
index 577ad6e..00ff61f 100644
--- a/pvmfw/src/main.rs
+++ b/pvmfw/src/main.rs
@@ -16,7 +16,6 @@
 
 #![no_main]
 #![no_std]
-#![feature(default_alloc_error_handler)]
 
 extern crate alloc;
 
diff --git a/rialto/src/main.rs b/rialto/src/main.rs
index 3b730f4..59ee0b6 100644
--- a/rialto/src/main.rs
+++ b/rialto/src/main.rs
@@ -16,7 +16,6 @@
 
 #![no_main]
 #![no_std]
-#![feature(default_alloc_error_handler)]
 
 mod exceptions;
 
diff --git a/vmbase/example/src/main.rs b/vmbase/example/src/main.rs
index 3b0e9db..9ec2dc4 100644
--- a/vmbase/example/src/main.rs
+++ b/vmbase/example/src/main.rs
@@ -16,7 +16,6 @@
 
 #![no_main]
 #![no_std]
-#![feature(default_alloc_error_handler)]
 
 mod exceptions;
 mod layout;