arm64/pmap: Remove valid bit check from r365879 assertions

[scottph]

Summary

pmap_kextract might see a pte with the valid bit cleared while
another processor is performing a pmap_update_entry operation, so
don't check for that bit in the assertion.

MFC after: 1 week
Sponsored by: Ampere Computing, Inc.

[emaste]

See D26510

[scottph]

In D26519#590130, @emaste wrote:

See D26510

oops, sorry for the noise