libstdc++: Remove unnecessary "& 1" from year_month_day_last::day()

When year_month_day_last::day() was implemented, Dr. Matthias Kretz realised
that the operation "& 1" wasn't necessary but we did not patch it at that
time. This patch removes the unnecessary operation.

libstdc++-v3/ChangeLog:

	* include/std/chrono (year_month_day_last::day): Remove &1.