Cleanup flowconfig options that are on by default in metro

Reviewed By: panagosg7

Differential Revision: D108282334

fbshipit-source-id: 3c79d584d9d6a195169a40f772223be93e58633d

Author: SamChou19815

.flowconfig

@@ -11,10 +11,6 @@ emoji=true
 format.bracket_spacing=false
 
 munge_underscores=true
-enums=true
-casting_syntax=as
-experimental.pattern_matching=true
-experimental.allow_variance_keywords=true
 
 module.name_mapper='\(metro-[^/]*\)' -> '<PROJECT_ROOT>/packages/\1/src/index.js'