Revert "Replace desktop watchstar icon with mobile skin one"

Based on the discussion on T56307, this change is controversial
and not necessarily wanted. In summary:

* Green star looks out of place with blue Vector (Edokter, Esanders, Trevor)
* Clashes with WikiLove heart icon (Edokter)
* Flat design doesn't fit with Vector's gradient style (Rillke)
* Green means constructive in MediaWiki UI, however clicking the
  green star removes a page from your watchlist (Krinkle)

This reverts commit 56825d209e5c87a23e30a7aa3cb57b5a731ec2be.

Change-Id: I46f75a0eb83f9643fc0b51befe02aad4a84efc4e