Remove remaining usage of winuwp flag #103586
Description
This wasn't removed in #102172. We can cleanup this too.
https://github.com/flutter/engine/blob/c007b3c242fce467e913c7edf20365c8c1c3e8be/BUILD.gn#L224-L228
cc: @cbracken