butc: Remove dead initialisation

The 'bytesread' variable is set by all code paths, so don't bother
assigning a default value each time we pass through the loop

Caught by clang-analyzer

Change-Id: I8b79bc888ba23e2bcfa9befb5bf034db8f3ca3f8
Reviewed-on: http://gerrit.openafs.org/9259
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>