Base: Remove unnecessary UID separation of multi-process Browser

After looking closely at this, I realized that we've been running
all the service processes under separate user accounts even though
there's actually no need to.

Since we already use pledge() and unveil() to limit the scope and
access of these programs, separating them to another UID doesn't
achieve anything meaningful. So let's bring them back to the "anon"
user account and simplify things.

Programs affected:

- ImageDecoder
- RequestServer
- WebContent
- WebSocket

Longer term, I'd like for all of these to get spawned for the current
desktop user somehow, possibly by some kind of session manager, or
perhaps by the Browser program itself. But for now they remain under
SystemServer's control.

Base/etc/group

@@ -4,13 +4,9 @@ tty:x:2:
 phys:x:3:window,anon
 audio:x:4:anon
 utmp:x:5:
-lookup:x:10:request,websocket,anon
-request:x:11:webcontent,anon
+lookup:x:10:anon
 notify:x:12:anon
 window:x:13:anon,notify
 clipboard:x:14:anon,notify
-webcontent:x:15:anon
-image:x:16:anon,webcontent
 symbol:x:17:anon
-websocket:x:18:webcontent,anon
 users:x:100:anon