Merge similar strings - Import & Export

Import &amp; Export
admin/class-pointers.php:163

Import & Export
admin/class-admin.php:164
inc/wpseo-non-ajax-functions.php:467

(less i18n strings, less work for the translations teams, smaller .MO/.PO)
so we can change the two other string in:
admin/class-admin.php:164
inc/wpseo-non-ajax-functions.php:467