xfce.xfce4-dev-tools: Remove unused xdtEnvHook

A `grep -nr envHooks` run shows that nothing picks up `envHooks`. automake's
setup-hook should already take care of `ACLOCAL_PATH`.

Fixes: 046f091e0d98

pkgs/desktops/xfce/core/xfce4-dev-tools/setup-hook.sh

···
-xdtEnvHook() {
-    addToSearchPath ACLOCAL_PATH $1/share/aclocal
-}
-
-envHooks+=(xdtEnvHook)
-
 xdtAutogenPhase() {
     mkdir -p m4
     NOCONFIGURE=1 xdt-autogen