osl: kill check for ancient freebsd < 7 versions

Change-Id: I082611edd88daae36181d62f9fc56b588630629b
Reviewed-on: https://gerrit.libreoffice.org/10828
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>