Fix issues brought up by CI.

- Remove trailing whitespace
- Remove unused variable.

plugins/disk/btrfs_device_stats

@@ -1,4 +1,4 @@
-#!/usr/bin/python3 
+#!/usr/bin/python3
 #
 # This file contains a munin-plugin to gather btrfs statistics per device.
 #
@@ -81,7 +81,6 @@ def munin_values(fs):
 
     for this_device in devices:
         this_dev_info = fs.dev_info(this_device.devid)
-        this_dev_name = this_dev_info.path.replace('/dev/', '')
         this_dev_stat = fs.dev_stats(this_device.devid, False)
 
         corruption_errs = this_dev_stat.corruption_errs