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.

doc/riverctl.1.scd

@@ -259,26 +259,6 @@ A complete list may be found in _/usr/include/linux/input-event-codes.h_
 	- _on-output-change_: When a different output is focused, the cursor will be
 	  warped to its center.
 
-*opacity* _focused_ _unfocused_ _initial_ _step-size_ _delta-t_
-	Configure server-side opacity of views, including transition
-	animations. A value of 0.0 is fully transparent while 1.0 is fully
-	opaque. By default all views are fully opaque and there are no
-	animations.
-
-	- _focused_: opacity of focused views [0.0, 1.0]
-	- _unfocused_: opacity of unfocused views [0.0, 1.0]
-	- _initial_: opacity of views when they are created before immediately
-	  transitioning to either _focused_ or _unfocused_ [0.0, 1.0]
-	- _step-size_: opacity change per step [0.05, 1.0]
-	- _delta-t_: step time in milliseconds
-
-	A transition animation may occur when changing between states with
-	different opacity values configured. Instead of setting the view's
-	opacity to the value for the new state immediately, it is changed
-	incrementally in steps of _step-size_ every _delta-t_ milliseconds.
-	Setting _step-size_ to 1.0 disables transitions fully regardless of
-	the value of _delta-t_.
-
 *set-repeat* _rate_ _delay_
 	Set the keyboard repeat rate to _rate_ key repeats per second and
 	repeat delay to _delay_ milliseconds.