metro-config: Don't always assert import/require with unstable_enablePackageExports (#49780)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49780

Now that we're using a minimum Metro version of 0.81.3, Metro will dynamically assert `import` or `require` based on the style of the import at source.

Asserting both is likely to cause issues with `babel/runtime` and potentially other packages - see https://github.com/facebook/metro/pull/1447.

Changelog:
[Internal] metro-config: Assert import/require dynamically when using `unstable_enablePackageExports` with Metro.

Reviewed By: hoxyq

Differential Revision: D70473577

fbshipit-source-id: b3e6fbd78e2b6fc0f9a16ec3db2c19698848e6d1

packages/metro-config/src/index.flow.js

@@ -53,7 +53,7 @@ export function getDefaultConfig(projectRoot: string): ConfigT {
     resolver: {
       resolverMainFields: ['react-native', 'browser', 'main'],
       platforms: ['android', 'ios'],
-      unstable_conditionNames: ['require', 'import', 'react-native'],
+      unstable_conditionNames: ['react-native'],
     },
     serializer: {
       // Note: This option is overridden in cli-plugin-metro (getOverrideConfig)