kernel/Makefile: remove the useless CFLAGS_REMOVE_cgroup-debug.o

The file cgroup-debug.c had been removed from commit fe6934354f8e
(cgroups: move the cgroup debug subsys into cgroup.c to access internal state).
Remain the CFLAGS_REMOVE_cgroup-debug.o = $(CC_FLAGS_FTRACE)
useless in kernel/Makefile.

Signed-off-by: Li Bin <huawei.libin@huawei.com>
Acked-by: Zefan Li <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/Makefile

@@ -14,8 +14,7 @@ obj-y     = fork.o exec_domain.o panic.o \
 obj-$(CONFIG_MULTIUSER) += groups.o
 
 ifdef CONFIG_FUNCTION_TRACER
-# Do not trace debug files and internal ftrace files
-CFLAGS_REMOVE_cgroup-debug.o = $(CC_FLAGS_FTRACE)
+# Do not trace internal ftrace files
 CFLAGS_REMOVE_irq_work.o = $(CC_FLAGS_FTRACE)
 endif