Cleanup unused string (#516)

See microsoft/vscode#246520

Author: roblourens

src/extension/conversation/vscode-node/chatParticipants.ts

@@ -219,7 +219,6 @@ Learn more about [GitHub Copilot](https://docs.github.com/copilot/using-github-c
 		const markdownString = new vscode.MarkdownString(helpPostfix);
 		markdownString.isTrusted = { enabledCommands: ['inlineChat.start', 'github.copilot.open.walkthrough'] };
 		defaultAgent.helpTextPostfix = markdownString;
-		defaultAgent.helpTextVariablesPrefix = vscode.l10n.t('You can also help me understand your question by using the following variables to give me extra context:');
 
 		defaultAgent.additionalWelcomeMessage = this.additionalWelcomeMessage;
 		defaultAgent.titleProvider = this.instantiationService.createInstance(ChatTitleProvider);

src/extension/vscode.proposed.defaultChatParticipant.d.ts

@@ -39,11 +39,6 @@ declare module 'vscode' {
 		 */
 		helpTextPrefix?: string | MarkdownString;
 
-		/**
-		 * A string that will be added before the listing of chat variables in `/help`.
-		 */
-		helpTextVariablesPrefix?: string | MarkdownString;
-
 		/**
 		 * A string that will be appended after the listing of chat participants in `/help`.
 		 */