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.

completions/zsh/_riverctl

@@ -48,7 +48,6 @@ _riverctl_subcommands()
         'border-color-unfocused:Set the border color of unfocused views'
         'border-width:Set the border width to pixels'
         'focus-follows-cursor:Configure the focus behavior when moving cursor'
-        'opacity:Configure server-side opacity of views'
         'set-repeat:Set the keyboard repeat rate and repeat delay'
         'set-cursor-warp:Set the cursor warp mode.'
         'xcursor-theme:Set the xcursor theme'