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/bash/riverctl

@@ -42,7 +42,6 @@ function __riverctl_completion ()
 			border-color-unfocused \
 			border-width \
 			focus-follows-cursor \
-			opacity \
 			set-repeat \
 			set-cursor-warp \
 			xcursor-theme"