river: remove opacity command

This code is complex and increases maintenance burden but doesn't
add any functionality, only eye-candy.

Futhermore, neither I nor any of the core contributors use it.

There may be a place in river for such eye-candy down the line, in which
case this code could be revived. Currently river is early enough in its
development that our focus should be on core functionality instead.

example/init

@@ -150,9 +150,6 @@ riverctl float-filter-add "popup"
 # Set app-ids of views which should use client side decorations
 riverctl csd-filter-add "gedit"
 
-# Set opacity and fade effect
-# riverctl opacity 1.0 0.75 0.0 0.1 20
-
 # Set and exec into the default layout generator, rivertile.
 # River will send the process group of the init executable SIGTERM on exit.
 riverctl default-layout rivertile