Drop procolor.js and the color picker based on it

Because:

• it's not our code so it shouldn't have been added to the source tree in the first place
• it's not used in XWiki Standard. We have another (Bootstrap-based) color picker implementation provided by the Flamingo skin
• it depends on Prototype.js which we want to get rid of
• it doesn't work in strict mode (XWIKI-18106) and it doesn't make sense to maintain some code that we're not using