flow: Stop pointing to old location of a RN-internal file.

Part of the RN v0.61 -> v0.62 changes to the template app [1],
corresponding to the two deletions in the Flow config in
facebook/react-native@e54ecf907.

We pointed to the file's new location in a recent commit before the
main upgrade commit; now, we finish following
facebook/react-native@e54ecf907 by removing the references to the
old, now-incorrect location of the file.

[1] https://react-native-community.github.io/upgrade-helper/?from=0.61.5&to=0.62.2

.flowconfig

@@ -45,7 +45,6 @@ node_modules/zulip-markdown-parser/
 
 
 [libs]
-node_modules/react-native/Libraries/react-native/react-native-interface.js
 node_modules/react-native/interface.js
 node_modules/react-native/flow/
 
@@ -106,7 +105,6 @@ esproposal.nullish_coalescing=enable
 
 munge_underscores=true
 
-module.name_mapper='^react-native$' -> '<PROJECT_ROOT>/node_modules/react-native/Libraries/react-native/react-native-implementation'
 module.name_mapper='^react-native/\(.*\)$' -> '<PROJECT_ROOT>/node_modules/react-native/\1'
 module.name_mapper='^@?[./a-zA-Z0-9$_-]+\.\(bmp\|gif\|jpg\|jpeg\|png\|psd\|svg\|webp\|m4v\|mov\|mp4\|mpeg\|mpg\|webm\|aac\|aiff\|caf\|m4a\|mp3\|wav\|html\|pdf\)$' -> '<PROJECT_ROOT>/node_modules/react-native/Libraries/Image/RelativeImageStub'