Remove unused rustbuild feature

I can't tell if this was ever used, but it's not used today.

Cargo.toml

@@ -17,7 +17,6 @@ c = ["gcc"]
 compiler-builtins = []
 default = ["compiler-builtins"]
 mem = []
-rustbuild = ["compiler-builtins"]
 mangled-names = []
 
 # generate tests