[ASImageNode] No longer uses placeholderColor

[master-nevi]

Version with issue: ASDK v2+ to master

Original thread from Slack channel: https://asyncdisplaykit.slack.com/archives/general/p1483578622003990

This prevents the placeholder API ([ASDisplayNode placeholderEnabled/placeholderFadeDuration/placeholderImage]) from being used to fade in remote images from a solid color after the download is finished. When looking at the ASImageNode it appears that placeholderColor is no longer used anywhere in the implementation.

[Adlai-Holler]

Resolved in #2866. Thanks for the issue report and building the fix @master-nevi !