[yam] Remove extractor

mymedia.yam.com is dead. An wikipedia user also pointed out that Yam's
blog service is no longer available. [1]

[1] https://zh.wikipedia.org/zh-tw/%E5%A4%A9%E7%A9%BA%E9%83%A8%E8%90%BD

youtube_dl/extractor/extractors.py

@@ -1280,7 +1280,6 @@ from .yahoo import (
     YahooIE,
     YahooSearchIE,
 )
-from .yam import YamIE
 from .yandexmusic import (
     YandexMusicTrackIE,
     YandexMusicAlbumIE,