Rollup merge of rust-lang#34081 - RustOS-Fork-Holding-Ground:no-core-build-script, r=alexcrichton

No build.rs for libcore

I did a grep and there are no longer any mention of "rustbuild" in core, in `cfg`s or otherwise.

Author: eddyb

src/libcore/Cargo.toml

@@ -2,7 +2,6 @@
 authors = ["The Rust Project Developers"]
 name = "core"
 version = "0.0.0"
-build = "build.rs"
 
 [lib]
 name = "core"