breaking: remove ios platform code from plugin repo (#263)

* breaking: remove iOS platform code from plugin repo
* chore: update README.md, remove iOS legacy SplashScreen information
* chore: simplify local browser tests

Note: Even though the implementation of the iOS SplashScreen is integrated into the core of cordova-ios, the documentation is left in this repo. This is because all of the SplashScreen documentation is grouped together on cordova.io and that documentation is pulled from this repo.

package.json

@@ -8,7 +8,6 @@
     "platforms": [
       "android",
       "ubuntu",
-      "ios",
       "windows",
       "browser"
     ]
@@ -20,7 +19,6 @@
     "splashscreen",
     "ecosystem:cordova",
     "cordova-android",
-    "cordova-ios",
     "cordova-windows"
   ],
   "scripts": {
@@ -36,9 +34,6 @@
         "cordova-android": ">=3.6.0",
         "cordova-windows": ">=4.4.0"
       },
-      "<6.0.0": {
-        "cordova-ios": "<6.0.0"
-      },
       "6.0.0": {
         "cordova": ">100"
       }