ima: Don't remove security.ima if file must not be appraised

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

---------------------------

Files might come from a remote source and might have xattrs, including
security.ima. It should not be IMA task to decide whether security.ima
should be kept or not. This patch removes the removexattr() system
call in ima_inode_post_setattr().

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>