remove unused variable

plugins/keepkey/plugin.py

@@ -1,5 +1,3 @@
-import threading
-
 from binascii import hexlify, unhexlify
 
 from electrum.util import bfh, bh2u
@@ -72,8 +70,6 @@ class KeepKeyCompatiblePlugin(HW_PluginBase):
 
     def __init__(self, parent, config, name):
         HW_PluginBase.__init__(self, parent, config, name)
-        self.main_thread = threading.current_thread()
-        # FIXME: move to base class when Ledger is fixed
         if self.libraries_available:
             self.device_manager().register_devices(self.DEVICE_IDS)