Fix error on browser extension import

[ror3d]

This implements the solution suggested by JabRef/JabRef-Browser-Extension#493 (comment)

Closes JabRef/JabRef-Browser-Extension#493

I found the issue to be the Remove-Item call, because the temporary file is being accessed by the running JabRef instance, so it results in the error message:

Cannot remove item ...\AppData\Local\Temp\1\tmp7642.tmp: The process cannot access the file '...\AppData\Local\Temp\1\tmp7642.tmp' because it is being used by another process.

By redirecting the output of that to $null, that error is not sent to the plugin and so the issue is patched.

I will create a separate issue in this repository to track the remaining problem of the temporary files not being deleted when the script is run.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[tobiasdiez]

Thanks for your PR!