Remove backreferences warning (#84092)

* Fallback to PCRE2 when backreferences are detected
And remove warning that we don't support them.

* Remove explicit fallback... ripgrep will do it implicitly

* Remove unused

src/vs/workbench/contrib/search/browser/searchWidget.ts

@@ -15,7 +15,6 @@ import { Action } from 'vs/base/common/actions';
 import { Delayer } from 'vs/base/common/async';
 import { Emitter, Event } from 'vs/base/common/event';
 import { KeyCode, KeyMod } from 'vs/base/common/keyCodes';
-import * as strings from 'vs/base/common/strings';
 import { CONTEXT_FIND_WIDGET_NOT_VISIBLE } from 'vs/editor/contrib/find/findModel';
 import * as nls from 'vs/nls';
 import { IClipboardService } from 'vs/platform/clipboard/common/clipboardService';
@@ -443,12 +442,6 @@ export class SearchWidget extends Widget {
 			return { content: e.message };
 		}
 
-		if (strings.regExpContainsBackreference(value)) {
-			if (!this.searchConfiguration.usePCRE2) {
-				return { content: nls.localize('regexp.backreferenceValidationFailure', "Backreferences are not supported") };
-			}
-		}
-
 		return null;
 	}