Remove abandoned WebKitAdditionsFeature1 flag, since it ended up not being needed and just complicates the build and generated preferences.
<rdar://problem/93353298>
Created attachment 459435 [details] Patch
Committed r294265 (250613@main): <https://commits.webkit.org/250613@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 459435 [details].