cred: remove unsued extern declaration change_create_files_as()

Since commit 3a3b7ce9 ("CRED: Allow kernel services to override LSM settings for task actions")
this is never used, so can be removed.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Fixes: 3a3b7ce9 ("CRED: Allow kernel services to override LSM settings for task actions")
[PM: subject tweak, fixes tag]
Signed-off-by: Paul Moore <paul@paul-moore.com>