remove unused variable

[knight9999]

Description

This PR is simply removing unnecessary codes from PushPlugin.m.

Related Issue

None

Motivation and Context

The plugin looks more complicacted because of unnecessary codes.

How Has This Been Tested?

I tests as follows.

1. getting registration id.
2. receive notification when app is background.
3. receive notification when app is foreground.
4. receive silent notification when app is background.
5. receive silent notification when app is foreground.

All functions work well as before.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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