fix: delete duplicated CharInputMapping entry in defaultInputMappings

[ChessNineeee]

Description

Describe your changes in detail
Delete the duplicated CharInputMapping entry in defaultInputMappings

Your issue description (body) goes here
https://github.com/contour-terminal/contour/issues/1635

Motivation and Context

Why is this change required? What problem does it solve?
It fixes the problem that copied text will be pasted twice.

How Has This Been Tested?

• Please describe how you tested your changes.
• see how your change affects other areas of the code, etc.

Checklist:

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• I have read the CONTRIBUTING document in my spoken language, and understand the terms
• I have updated (or added) the documentation accordingly.
• I have added tests to cover my changes.
• I have gone through all the steps, and have thoroughly read the instructions

[Yaraslaut]

Thanks a lot @ChessNineeee
PR refs #1635