selftests: fix timestamping Makefile

The clean target in the makefile conflicts with the generic
kselftests lib.mk, and fails to properly remove the compiled
test programs.

Remove the redundant rule, the TEST_GEN_FILES will be already
removed by the CLEAN macro in lib.mk.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Acked-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>