Replace @nda directive with resolver helper

Export and request-access already know visibility and
auth; a shared requireCompletedNDA call keeps NDA
enforcement there and drops GraphQL PUBLIC probing.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/server/api/complianceportal/v1/graphql/base.graphql

@@ -11,8 +11,6 @@ directive @goModel(
 
 directive @goEnum(value: String) on ENUM_VALUE
 
-directive @nda on FIELD_DEFINITION | OBJECT
-
 scalar BigInt
 scalar CursorKey
 scalar Datetime