Drop Summer and Winter time (DST)

[stelgenhof]

Yasumi presently includes entries for Summertime (DST) and Wintertime for the Netherlands and Denmark providers. Unfortunately, the estimation of historical summertime transitions, particularly between 1912 and 1946, is inconsistent among PHP and OS versions. I suppose there are two causes behind this:

• The underlying TimeZone database version installed with an operating system may vary.
• A newer version of a TimeZone database may include retroactive corrections as new (historic) information emerges that advocates for those corrections.

Supporting/maintaining DST gets difficult since users make use of Yasumi with various PHP versions and operating systems. Furthermore, we can argue whether Summertime and Wintertime are truly "holidays" in the way that Yasumi defines holidays.

For these reasons, I will drop the use of Summertime and Wintertime in Denmark and the Netherlands.