Remove unnecessary flag when running JS server

[thecodrr]

Summary

This removes the --projectRoot flag in launchPackager.bat as it was removed, fixing this bundler not opening issue on cli repo when yarn react-native run-android is run.

This is Windows related only so should be only tested on Windows.

Changelog

[Internal] [Fixed] - Fixed Metro Bundler not opening when running yarn react-native run-android

Test Plan

1. Create a new repo using react-native init
2. Edit the launchPackager.bat file
3. Run yarn react-native run-android

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[thymikee]

@theweavrs please check this: react-native-community/cli#484 (comment) looks like we need another solution

[victorwpbastos]

You should see react-native-community/cli#484 (comment) for clarification.

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[cpojer]

👍

[facebook-github-bot]

@cpojer is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @theweavrs in a162554.

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