Makefile: fix distclean target

Makefile: fix distclean target

As some Makefiles in sub directories do not implement the distclean
target, namely:
PVE/Service/Makefile
PVE/CLI/Makefile

This target is broken.

As all other implementations just redirect to the 'clean' target I
do not implement the missing ones but rather remove all such
targets. Keep it just in the top level directory, for consistence
sake with other pve repos, and redirect it there directly to the
clean target.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>