doc: remove ignoreSelfRefs from docs

While working on the LocalDerivationGoal code, I realized that this
attribute is only set to `false`/`true` depending on whether
`__structuredAttrs` is `true`/`false`.

Change-Id: I53868cd32cedd7e25cb6233bd93bc01111b56a07

doc/manual/src/language/advanced-attributes.md

@@ -302,8 +302,6 @@ Derivations can declare some infrequently used optional attributes.
 
     - `maxSize` defines the maximum size of the resulting [store object](../glossary.md#gloss-store-object).
     - `maxClosureSize` defines the maximum size of the output's closure.
-    - `ignoreSelfRefs` controls whether self-references should be considered when
-      checking for allowed references/requisites.
 
     Example: