Remove unnecessary line

Author: Vasileios Karakasis

README.md

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 [![Slack](https://reframe-slack.herokuapp.com/badge.svg)](https://reframe-slack.herokuapp.com/)<br/>
 [![License](https://img.shields.io/badge/License-BSD%203--Clause-blue.svg)](https://opensource.org/licenses/BSD-3-Clause)
 
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 # ReFrame
 
 ReFrame is a framework for writing regression tests for HPC systems.