Delete unused term "client-side discoverable credential property"

Fixes this Bikeshed lint:

```
LINT: Unexported dfn that's not referenced locally - did you mean to export it?
<dfn data-dfn-type="dfn" data-dfn-for="CredentialPropertiesOutput" id="credentialpropertiesoutput-client-side-discoverable-credential-property" data-lt="client-side discoverable credential property" data-noexport="by-default">client-side discoverable credential property<a href="#credentialpropertiesoutput-client-side-discoverable-credential-property" class="self-link"></a></dfn>
<dfn data-dfn-type="dfn" data-dfn-for="CredentialPropertiesOutput" id="credentialpropertiesoutput-client-side-discoverable-credential-property" data-lt="client-side discoverable credential property" data-noexport="by-default">client-side discoverable credential property<a href="#credentialpropertiesoutput-client-side-discoverable-credential-property" class="self-link"></a></dfn>
```

index.bs

@@ -6813,8 +6813,7 @@ This [=client extension|client=] [=registration extension=] and [=authentication
 
     <div dfn-type="dict-member" dfn-for="CredentialPropertiesOutput">
         :   <dfn>rk</dfn>
-        ::  This OPTIONAL property, known abstractly as the <dfn dfn-type="dfn">resident key credential property</dfn>
-            (i.e., <dfn dfn-type="dfn">client-side discoverable credential property</dfn>),
+        ::  This OPTIONAL property, known abstractly as the <dfn dfn-type="dfn">resident key credential property</dfn>,
             is a Boolean value indicating whether the {{PublicKeyCredential}} returned as a result of a [=registration ceremony=]
             is a [=client-side discoverable credential=].
             If {{rk}} is [TRUE], the credential is a [=discoverable credential=].