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 <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>
Reported-by: John Mcnamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>
Signed-off-by: Thomas Monjalon <ee2fa8b2681c737f775687e475dcbf2028a412f1@6wind.com>

venkynv/dpdk-mirror,msune/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,mixja/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,mixja/dpdk,mixja/dpdk,msune/dpdk,msune/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,mixja/dpdk,msune/dpdk,tsphillips/dpdk-fork,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror