style-guide: Remove inaccurate statement about rustfmt

rustfmt does include a mechanism to distinguish standard library
imports, which it does syntactically by crate name. Avoid making a
misleading statement that implies it cannot do this.

Author: joshtriplett

src/doc/style-guide/src/items.md

@@ -505,10 +505,6 @@ use b;
 Because of `macro_use`, attributes must also start a new group and prevent
 re-ordering.
 
-Note that tools which only have access to syntax (such as `rustfmt`) cannot
-tell which imports are from an external crate or the std lib, etc.
-
-
 #### Ordering list import
 
 Names in a list import must be sorted ascii-betically, but with `self` and