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>

tools/testing/selftests/ptp/Makefile

 # SPDX-License-Identifier: GPL-2.0
 CFLAGS += $(KHDR_INCLUDES)
-TEST_PROGS := testptp
+TEST_GEN_PROGS := testptp
 LDLIBS += -lrt
-all: $(TEST_PROGS)
+TEST_PROGS = phc.sh
 
 include ../lib.mk
-
-clean:
-	rm -fr $(TEST_PROGS)