feat!: use XDG_DATA_HOME as AGENDA_DIR fallback

I first decided against this, but the old `~/.app-trash` is no more
fitting than the XDG_DATA_HOME location.

The xdg-user-dir DOCUMENTS fallback is too presumptuous and removing it
makes things even simpler.

completion.bash

@@ -5,11 +5,7 @@
 # TODO: Maybe its easier to add an api to the agenda script to query for the
 #       available -t option values?
 if [ -z "$AGENDA_DIR" ]; then
-	if type xdg-user-dir >/dev/null 2>&1; then
-		AGENDA_DIR="$(xdg-user-dir DOCUMENTS)/agenda"
-	else
-		AGENDA_DIR="$HOME/.agenda"
-	fi
+	AGENDA_DIR="${XDG_DATA_HOME:-"$HOME/.local/share"}/agenda"
 fi
 
 AGENDA_COMP_DATE_MOD=("next" "last")