Replace $Call in metro

Summary:
With the introduction of [new utility types](https://flow.org/en/docs/types/utilities/#toc-return-type) in Flow, these $Call are no longer needed

Changelog: [Internal]

Reviewed By: jbrown215

Differential Revision: D46862023

fbshipit-source-id: bae129e4fd85a44a07283bc012a0396213647fee

packages/metro/src/DeltaBundler/Serializers/sourceMapGenerator.js

@@ -26,8 +26,6 @@ const {
   fromRawMappingsNonBlocking,
 } = require('metro-source-map');
 
-type ReturnType<F> = $Call<<A, R>((...A) => R) => R, F>;
-
 function getSourceMapInfosImpl(
   isBlocking: boolean,
   onDone: ($ReadOnlyArray<ReturnType<typeof getSourceMapInfo>>) => void,