Delete an unwanted include (#7786)

Author: wu-hui

Firestore/Example/Tests/Integration/API/FIRBundlesTests.mm

@@ -23,7 +23,6 @@
 #import "Firestore/Example/Tests/Util/FSTHelpers.h"
 #import "Firestore/Example/Tests/Util/FSTIntegrationTestCase.h"
 
-#import "../../../../../../../../../Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/experimental/string"
 #include "Firestore/core/src/util/string_apple.h"
 #include "Firestore/core/test/unit/testutil/bundle_builder.h"