[lldb][test] Remove class level packetLog in MockGDBServerResponder (#162453)

Added in 1902ffd but appears to be
leftover code from some older design.

I can't find any code that reads packetLog via the class itself, or
checks whether it is None.

No tests failed on AArch64 Linux after removing it.

Author: DavidSpickett

lldb/packages/Python/lldbsuite/test/gdbclientutils.py

@@ -87,7 +87,6 @@ class MockGDBServerResponder:
     """
 
     registerCount = 40
-    packetLog = None
 
     class RESPONSE_DISCONNECT:
         pass