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: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/watchdog/Makefile

-TEST_PROGS := watchdog-test
-
-all: $(TEST_PROGS)
+TEST_GEN_PROGS := watchdog-test
 
 include ../lib.mk
-
-clean:
-	rm -fr $(TEST_PROGS)