river: remove system /etc dir from init search paths

A true "default" config doesn't make sense for river. Everyone who uses
river seriously will customize their init script. Futhermore, the
current behavior of embedding the install path of the default system
config in the river binary is complex and prone to breaking.

doc/river.1.scd

@@ -32,19 +32,17 @@ utility may be used to communicate with river over these protocols.
 
 # CONFIGURATION
 
-On startup river will look for and run an executable file at one of the
-following locations, checked in the order listed:
-
-- $XDG_CONFIG_HOME/river/init
-- $HOME/.config/river/init
-- /etc/river/init
+On startup river will run an executable file at `$XDG_CONFIG_HOME/river/init`
+if such an executable exists. If $XDG_CONFIG_HOME is not set, ~/.config/ will
+be used instead.
 
 The executable init file will be run as a process group leader after river's
 wayland server is initialized but before entering the main loop. On exit,
 river will send SIGTERM to this process group.
 
-Usually this will be a shell script invoking *riverctl*(1) to create mappings,
-start programs such as a status bar, and preform other configuration.
+Usually this executable is a shell script invoking *riverctl*(1) to create
+mappings, start programs such as a layout generator or status bar, and
+preform other configuration.
 
 # ENVIRONMENT