cs..types.image.gradient: Remove -ms- prefix

An abstract type having a prefix doesn't make any sense. Nevertheless, I
tested gradients on Edge 18 and 15 and found that prefixes had no effect
for `conic-gradient()` or `linear-conic-gradient()` and neither was
supported in any case.

I believe this be a case of a prefix being wrongly reported, then
copied, from IE data.

css/types/image.json

@@ -188,7 +188,6 @@
               ],
               "chrome_android": "mirror",
               "edge": {
-                "prefix": "-ms-",
                 "version_added": "12"
               },
               "firefox": {