docs: Removing spurious heading from Javadoc comment

There's an H3 at the top of the PorterDuff.Mode javadoc comment,
and that breaks the TOC in various ways. We don't need that
heading so I'm removing it.

Test: make ds-docs-java
Exempt-from-owner-approval: Docs-only change
Change-Id: I6d5f98b5103a77a7d0245a438b6961c4a21fb059
Bug: 311897143