Add libstd support to Rust BPF (#5788)

programs/bpf/rust/tick_height/src/lib.rs

@@ -1,12 +1,6 @@
 //! @brief Example Rust-based BPF program that prints out the parameters passed to it
 
-#![no_std]
-#![allow(unreachable_code)]
-
-#[cfg(not(test))]
-extern crate solana_sdk_bpf_no_std;
 extern crate solana_sdk_bpf_utils;
-
 use byteorder::{ByteOrder, LittleEndian};
 use solana_sdk_bpf_utils::entrypoint::*;
 use solana_sdk_bpf_utils::{entrypoint, info};