Remove mention of RFC 1270

Looking at the RFC, it doesn't seem like it offers any particular
insight or motivation that isn't already clear from what the attribute
does.

We still need a policy for how we refer to historical information like
this, but generally I'd lean away from it.

Author: ehuss

src/attributes/diagnostics.md

@@ -330,13 +330,6 @@ The `deprecated` attribute may be applied to any [item], [trait item], [enum var
 Tuple struct fields are ignored.
 -->
 
-
-
-
-The [RFC][1270-deprecation.md] contains motivations and more details.
-
-[1270-deprecation.md]: https://github.com/rust-lang/rfcs/blob/master/text/1270-deprecation.md
-
 r[attributes.diagnostics.must_use]
 ## The `must_use` attribute