docs: remove old browser checks

Author: redonkulus

RELEASE.md

@@ -31,19 +31,13 @@ This assumes the following repo's are cloned and `npm` installed:
   - [ ] Chrome
   - [ ] Firefox
   - [ ] Safari
-  - [ ] iOS 10.x
-  - [ ] Android 4.x
-  - [ ] Android 5.x
 
 - [ ] **Review pure-site in target environments with [Pure served locally](https://github.com/pure-css/pure-site/blob/master/README.md#running-with-pure-served-locally)**
 
   - [ ] Edge
   - [ ] Chrome
   - [ ] Firefox
   - [ ] Safari
-  - [ ] iOS 10.x
-  - [ ] Android 4.x
-  - [ ] Android 5.x
 
 - [ ] **Review HISTORY.md**