Remove unused variables

Global variable keyhandler is declared as pointer to incomplete struct
type. Upon closer inspection, it turned out that this variable and
client_handles_keys are unused.

debian/patches/07-reinit-on-any-error.diff

@@ -3,7 +3,7 @@ Description: Reinitialize on any libusb error.
 Author: Alexander Ponyatykh <lazyranma@gmail.com>
 Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/libusb/+bug/1357556
 Forwarded: no
-Last-Update: 2015-05-18
+Last-Update: 2019-11-17
 ===================================================================
 --- a/g15daemon/main.c
 +++ b/g15daemon/main.c