flowconfig [nfc]: Remove an ignore about node_modules/warning/.

`yarn why` tells me we don't have an NPM package called "warning",
so I can't imagine what this line might have been doing for us.

Anyway, done to follow the template-app change in
facebook/react-native@54e19a6b7, on the path to the RN v0.64
upgrade.

.flowconfig

@@ -7,10 +7,6 @@
 ; Ignore polyfills
 node_modules/react-native/Libraries/polyfills/.*
 
-; These should not be required directly
-; require from fbjs/lib instead: require('fbjs/lib/warning')
-node_modules/warning/.*
-
 ; Flow doesn't support platforms
 .*/Libraries/Utilities/LoadingView.js