remove double define of DATE_APP_GENERIC_MDY

This double define (like #1607) is also quite useless and can be removed.
The confusing code was most likely introduced in the first place to work around some problems that the form/date_time helper used to have with internationalized dates before #1604 was merged.

Now it's only used it these places, wich can perfectly deal with any date() format:

https://github.com/concrete5/concrete5/blob/master/web/concrete/core/helpers/date.php#L170
https://github.com/concrete5/concrete5/blob/master/web/concrete/elements/block_master_collection_alias.php#L39
https://github.com/concrete5/concrete5/blob/master/web/concrete/elements/block_master_collection_alias.php#L40