Revert "Fix SYSCONFDIR to include "prefix""

This reverts commit 6942f5b684.
SYSCONFDIR should be "/etc" even when prefix="/usr" to be FHS compliant.
This is the default in meson from v0.44.

meson.build

@@ -114,7 +114,7 @@ if scdoc.found()
 	endforeach
 endif
 
-add_project_arguments('-DSYSCONFDIR="/@0@/@1@"'.format(prefix, sysconfdir), language : 'c')
+add_project_arguments('-DSYSCONFDIR="/@0@"'.format(sysconfdir), language : 'c')
 
 version = get_option('sway-version')
 if version != ''