x86/p2m: clear_identity_p2m_entry() must cope with 'relaxed' RDM mode

x86/p2m: clear_identity_p2m_entry() must cope with 'relaxed' RDM mode

Tearing down a 1:1 mapping that was never established isn't really nice
(and in fact hits an ASSERT() in p2m_remove_page()). Convert from a
wrapper macro to a proper function which then can take care of the
situation.

Also take the opportunity to remove the 'page_order' parameter of
clear_identity_p2m_entry(), to make it match set_identity_p2m_entry().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>