Prevent Cache Expiration to be override when saving the Performance Settings Form

When saving the Performance Settings form the Global page expiration is being set to 0 which is causing the website cache to be turned off. This is happening because in form alter the 'page_cache_maximum_age' is being unset and the Core Performance form is checking if the value is set. So my proposal is to restrain only the access to the setting.

Leaving a patch that works.