[clangd] Remove vsc-extension-quickstart.md from the vscode-clangd plugin

[nridge]

We could go a step further and describe it as the "reference client" for clangd, but I'm not sure how accurate / desirable that is at this stage.

[hokein]

I think we can even delete this file, since this is boilerplate doc generated by the vscode extension generator.

[nridge]

Remove entire file

clang-tools-extra/clangd/clients/clangd-vscode/vsc-extension-quickstart.md

-# Toy VS Code Extension for clangd
-
-## What's in the folder
-* This folder contains all of the files necessary for your extension
-* `package.json` - this is the manifest file in which you declare your extension and command.
-The sample plugin registers a command and defines its title and command name. With this information
-VS Code can show the command in the command palette. It doesn’t yet need to load the plugin.
-* `src/extension.ts` - this is the main file where you will provide the implementation of your command.
-The file exports one function, `activate`, which is called the very first time your extension is
-activated (in this case by executing the command). Inside the `activate` function we call `registerCommand`.
-We pass the function containing the implementation of the command as the second parameter to
-`registerCommand`.
-
-## Get up and running straight away
-* press `F5` to open a new window with your extension loaded
-* run your command from the command palette by pressing (`Ctrl+Shift+P` or `Cmd+Shift+P` on Mac) and typing `Hello World`
-* set breakpoints in your code inside `src/extension.ts` to debug your extension
-* find output from your extension in the debug console
-
-## Make changes
-* you can relaunch the extension from the debug toolbar after changing code in `src/extension.ts`
-* you can also reload (`Ctrl+R` or `Cmd+R` on Mac) the VS Code window with your extension to load your changes
-
-## Explore the API
-* you can open the full set of our API when you open the file `node_modules/vscode/vscode.d.ts`
-
-## Run tests
-* open the debug viewlet (`Ctrl+Shift+D` or `Cmd+Shift+D` on Mac) and from the launch configuration dropdown pick `Launch Tests`
-* press `F5` to run the tests in a new window with your extension loaded
-* see the output of the test result in the debug console
-* make changes to `test/extension.test.ts` or create new test files inside the `test` folder
-    * by convention, the test runner will only consider files matching the name pattern `**.test.ts`
-    * you can create folders inside the `test` folder to structure your tests any way you want