parser: Deprecate string type for $lang arg in DateFormatter::getInstance

parser: Deprecate string type for $lang arg in DateFormatter::getInstance

The parameter $lang is not used with type string in the Git repository:
https://codesearch.wmflabs.org/search/?q=DateFormatter%3A%3AgetInstance
This function is only used with type Language and without parameter.

The support for the language as string will be removed with Change-ID
I677ab561d67b63f0c86f65fadf9319e41444a22a.

The removal of the support for the type string simplifies the code.

Change-Id: I0c0ab46ffc0fab6b68672940e979ad7e5754a7d1