ssh_config.5: Remove redundant CheckHostIP default text

In 2000 (commit a95c1225217b) we changed the CheckHostIP default to
"no". We added text to ssh_config(5) documenting FreeBSD's default.

In 2021 OpenSSH made the same change, released with OpenSSH 8.5p1.
When we imported the update the added text remained, resulting in:

If the option is set to no (the default), the check will not be
executed.  The default is no.

Remove the now-redundant text.

Fixes: 206be79acbde ("Vendor import of OpenSSH 8.5p1")
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 06016adaccca1958cdde4edf845f5b972be7ffc0)
(cherry picked from commit 2c97e333166d4f6f6b84da60b96a82982bb19649)