selftests/bpf: Drop return in bpf_testmod_exit

bpf_testmod_exit() does not need to have a return value (given the void),
so this patch drops this useless 'return' in it.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/5765b287ea088f0c820f2a834faf9b20fb2f8215.1706442113.git.tanggeliang@kylinos.cn

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -619,7 +619,7 @@ static void bpf_testmod_exit(void)
 	while (refcount_read(&prog_test_struct.cnt) > 1)
 		msleep(20);
 
-	return sysfs_remove_bin_file(kernel_kobj, &bin_attr_bpf_testmod_file);
+	sysfs_remove_bin_file(kernel_kobj, &bin_attr_bpf_testmod_file);
 }
 
 module_init(bpf_testmod_init);