Removed duplicate "longer" MIGRATION_GUIDE.md

[Sikandar4747]

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Description

One Line Summary

Removed duplicate "longer" MIGRATION_GUIDE.md

Details

Motivation

REQUIRED - This ensures better readability for the one signal flutter docs.

Scope

RECOMMEND - OPTIONAL - Migration guide docs.

Affected code checklist

• [ ] Notifications
  • [ ] Display
  • [ ] Open
  • [ ] Push Processing
  • [ ] Confirm Deliveries
• [ ] Outcomes
• [ ] Sessions
• [ ] In-App Messaging
• [ ] REST API requests
• [ ] Public API changes

Checklist

Overview

• [x] I have filled out all REQUIRED sections above
• [x] PR does one thing
  • If it is hard to explain how any codes changes are related to each other then it most likely needs to be more than one PR
• [x] Any Public API changes are explained in the PR details and conform to existing APIs

Testing

• [] I have included test coverage for these changes, or explained why they are not needed
• [ ] All automated tests pass, or I explained why that is not possible
• [ ] I have personally tested this on my device, or explained why that is not possible

Final pass

• [x] Code is as readable as possible.
  • Simplify with less code, followed by splitting up code into well named functions and variables, followed by adding comments to the code.
• [x] I have reviewed this PR myself, ensuring it meets each checklist item
  • WIP (Work In Progress) is ok, but explain what is still in progress and what you would like feedback on. Start the PR title with "WIP" to indicate this.

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