remove DATE_APP_GENERIC_MDY definition

I think this definition still doesn't make any sense and should be removed. The way it is implemented now means

en_US always leads to "n/j/Y"
any other locale always leads to "Y-m-d"

But what you really want is make DATE_APP_GENERIC_MDY translatable, right?

Still, I am confused how the DATE_APP_DATE_PICKER fits into this... 
See this old PR: https://github.com/concrete5/concrete5/pull/659