Remove `external` field from Releases and Release Links APIs

Overview

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

As such, the external field in Releases and Release Links APIs was no longed needed, and was deprecated in %15.9.

This issue tracks the removal of external field.

Proposal

• Remove the external field in Releases API.
• Remove the external field in Release Links API.
• Remove any references to external field in the documentation.