Remove old vendor prefixes

[Rami Yushuvaev]

See this test result: https://yellowlab.tools/result/fy21f9wpqw/rule/cssOldPropertyPrefixes

Many property prefixes such as -moz- or -webkit- are not needed anymore, or by very few people. Sometimes, they have never even existed. We can remove them or replace them with the non-prefixed version. This will help reducing your stylesheets weight.

TODO: update the css-prefixes script in package.json.