[lldb][test] Turn ObjC string literals to C-style literals (NFC) (llvm#69793) (#7998)

The underlying timezone classes are being reimplemented in Swift, and these 
strings will be Swift strings, without the ObjC `@` prefix. Leaving off the `@` 
makes these tests usable both before and after the reimplmentation of 
Foundation in Swift.

Author: kastiglione

lldb/test/API/functionalities/data-formatter/data-formatter-objc/TestDataFormatterObjCNSDate.py

@@ -53,12 +53,12 @@ def nsdate_data_formatter_commands(self):
 
         self.expect(
             "frame variable cupertino home europe",
-            substrs=['@"America/Los_Angeles"', '@"Europe/Rome"', '@"Europe/Paris"'],
+            substrs=['"America/Los_Angeles"', '"Europe/Rome"', '"Europe/Paris"'],
         )
 
         self.expect(
             "frame variable cupertino_ns home_ns europe_ns",
-            substrs=['@"America/Los_Angeles"', '@"Europe/Rome"', '@"Europe/Paris"'],
+            substrs=['"America/Los_Angeles"', '"Europe/Rome"', '"Europe/Paris"'],
         )
 
         self.expect(