Don't implicitly talk about free constant CTFE timing

@RalfJung pointed out in a [comment] that the previous phrasing of the
sentence can read like it is giving guarantees about free constant
definitions always undergoing CTFE, even when unused. That seems to be
how the compiler behaves right now, but it's unclear whether it's
intentional.

Be more precise and don't talk about free constants at all.

[comment]: rust-lang#1120 (comment)

src/items/associated-items.md

@@ -293,8 +293,8 @@ type that the definition has to implement.
 An *associated constant definition* defines a constant associated with a
 type. It is written the same as a [constant item].
 
-Unlike [free] constants, associated constant definitions undergo
-[constant evaluation] only when referenced.
+Associated constant definitions undergo [constant evaluation] only when
+referenced.
 
 ```rust
 struct Struct;
@@ -381,5 +381,4 @@ fn main() {
 [regular function parameters]: functions.md#attributes-on-function-parameters
 [generic parameters]: generics.md
 [where clauses]: generics.md#where-clauses
-[free]: ../glossary.md#free-item
 [constant evaluation]: ../const_eval.md