selftests: media_tests: Fix Makefile 'clean' target warning

Remove 'clean' target and change TEST_PROGS to TEST_GEN_PROGS so the
common lib.mk 'clean' target clean these generated files. TEST_PROGS
is for shell scripts and not for generated test executables.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/media_tests/Makefile

 # SPDX-License-Identifier: GPL-2.0
-TEST_PROGS := media_device_test media_device_open video_device_test
-all: $(TEST_PROGS)
+TEST_GEN_PROGS := media_device_test media_device_open video_device_test
+all: $(TEST_GEN_PROGS)
 
 include ../lib.mk
-
-clean:
-	rm -fr media_device_test media_device_open video_device_test