mk: revert "allow application to override clean"

mk: revert "allow application to override clean"

This reverts commit ca0038c0094dd8a63fc7595e26d0071a3e3bd72e.

It forces clean target of application to have double-colon.
As stated in make doc:
	"Double-colon rules are somewhat obscure and not often very useful"
And it's not needed since POSTCLEAN allow to add some clean targets.

Reported-by: Olivier Matz <olivier.matz@6wind.com>
Reported-by: John Mcnamara <john.mcnamara@intel.com>
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>