Remove `external` field in GraphQL ReleaseAssetLink type

Overview

We started to treat all release links as external in %15.9. See #333967 (closed).

As such, the external field in ReleaseAssetLink type is no longer needed, and was deprecated in %15.9.

This issue tracks the removal of external field in GraphQL.

Proposal

• Remove the external field in GraphQL ReleaseAssetLink type.
• Remove any references to external field in the GraphQL documentation.