flow: Handle HMRLoadingView -> LoadingView rename (2/2).

Part of the RN v0.60 -> v0.61 upgrade [1]. Corresponds to
facebook/react-native@ba8f88d1a.

Before the upgrade commit, we added the (then-unnecessary) new name.
Now that the upgrade is done, we remove the (now-unnecessary) old
name.

.flowconfig

@@ -18,7 +18,6 @@ node_modules/react-native/Libraries/polyfills/.*
 node_modules/warning/.*
 
 ; Flow doesn't support platforms
-.*/Libraries/Utilities/HMRLoadingView.js
 .*/Libraries/Utilities/LoadingView.js
 
 ; Custom ignores