selftests/ptp: Remove clean target from Makefile

Fix the following build warn removing unnecessary clean target
from the Makefile. lib.mk handles clean.

Makefile:10: warning: overriding recipe for target clean
../lib.mk:124: warning: ignoring old recipe for target clean

In addition, fix to use TEST_GEN_PROGS for generated test executables
and TES_PROGS for the shell script. Ger rid of all target as lib.mk
handles it.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>