Bug 1550596: Remove overly broad keep rule from GV proguard rules. r=snorp,agi

Requesting reviewers based on `hg blame` output and general knowledge of who is working on the project. I hope that's okay.

Differential Revision: https://phabricator.services.mozilla.com/D30580

--HG--
extra : moz-landing-system : lando

mobile/android/geckoview/proguard-rules.txt

@@ -4,13 +4,6 @@
 
 -keepattributes *Annotation*
 
-# Preserve all public classes, and their public and protected fields and
-# methods.
-
--keep public class * {
-    static public protected *;
-}
-
 # Preserve all .class method names.
 
 -keepclassmembernames class * {