selftests: watchdog: fix to use TEST_GEN_PROGS and remove clean

TEST_PROGS should be used for test scripts that don't ned to be built.
Use TEST_GEN_PROGS instead which is intended for test executables.

Remove clean target and let the common clean take care of cleaning.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>