security_selinux: Drop @optional from _virSecuritySELinuxContextItem

security_selinux: Drop @optional from _virSecuritySELinuxContextItem

Now, that we don't need to remember if setting context is
'optional' (the argument only made
virSecuritySELinuxSetFileconImpl() return a different success
code), we can drop it from the _virSecuritySELinuxContextItem
structure as we don't need to remember it in transactions.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>