[cleanup] Simplify FormatDate
[cleanup] Simplify FormatDate

'Day_' + enMonthNames is already given by dayMnthFmts list

Change-Id: Iac2aa2ab3a08004fb10e79cd9098ddb54a4796b1

wikimedia/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core