fix: removed duplicated sentence in Key Concepts section

Remove duplicate occurence of "Each AI application implements the client side of MCP once, and each tool/data source implements the server side once."

Author: nilsreck

units/en/unit1/key-concepts.mdx

@@ -31,8 +31,6 @@ MCP transforms this into an M+N problem by providing a standard interface: each
 
 ![With MCP](https://huggingface.co/datasets/mcp-course/images/resolve/main/unit1/2.png)
 
-Each AI application implements the client side of MCP once, and each tool/data source implements the server side once.
-
 ## Core MCP Terminology
 
 Now that we understand the problem that MCP solves, let's dive into the core terminology and concepts that make up the MCP protocol.