docs(jetbrains-plugin): remove incorrect Protocol Buffer plugin requirement

The Protocol Buffer plugin is not a dependency of this plugin and is not
required for the gRPC Federation plugin to function. The LSP server handles
all proto file processing independently.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: utahta

lsp/client/jetbrains/README.md

@@ -13,7 +13,6 @@ The gRPC Federation Language Server for JetBrains is a plugin designed for JetBr
 
 - **IntelliJ IDEA Ultimate** 2023.3 or newer (This plugin requires Ultimate Edition)
 - **grpc-federation-language-server** installed and available in PATH
-- **Protocol Buffer** plugin (usually pre-installed)
 
 ## Installation