Removed the S3 and WebDAV build flags

So these special remotes are always supported.

IIRC these build flags were added because the dep chains were a bit too
long, or perhaps because the libraries were not available in Debian stable,
or something like that. That was long ago, those reasons no longer apply,
and users get confused when builtin special remotes are not available, so
it seems best to remove the build flags now.

If this does cause a problem it can be reverted of course..

This commit was sponsored by Jochen Bartl on Patreon.

standalone/osx/stack.yaml

@@ -3,8 +3,6 @@ flags:
     production: true
     assistant: true
     pairing: true
-    s3: true
-    webdav: true
     torrentparser: true
     webapp: true
     magicmime: true