Update README.md to remove legacy flag (#206)

Author: aaaronic

README.md

@@ -4,13 +4,11 @@
 
 This crate is intended for working with the GBA.
 
-To build for the GBA you'll need to use `build-std` and you'll also need to
-activate the `compiler-builtins-weak-intrinsics` feature.
+To build for the GBA you'll need to use `build-std`.
 
 The following should be somewhere in your `.cargo/config.toml`:
 
 ```toml
 [unstable]
 build-std = ["core"]
-build-std-features = ["compiler-builtins-weak-intrinsics"]
 ```