ima: don't remove the securityfs policy file

The securityfs policy file is removed unless additional rules can be
appended to the IMA policy (CONFIG_IMA_WRITE_POLICY), regardless as
to whether the policy is configured so that it can be displayed.

This patch changes this behavior, removing the securityfs policy file,
only if CONFIG_IMA_READ_POLICY is also not enabled.

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>