Remove redundant instructions

We just need to install bpf-linker; the rest of this is already encoded
in rust-toolchain.toml.

Fixes #83.

README.md

@@ -2,8 +2,6 @@
 
 ## Prerequisites
 
-1. Install a rust stable toolchain: `rustup install stable`
-1. Install a rust nightly toolchain with the rust-src component: `rustup toolchain install nightly --component rust-src`
 1. Install bpf-linker: `cargo install bpf-linker`
 
 ## Build eBPF