When sending SIGUSR1 to the daemon, it can crash because of a concurrent
map access panic, showing a stack trace involving dumpDaemon. It appears
it's not possible to recover from a concurrent map access panic. Since
it's important that SIGUSR1 not be a destructive operation, sadly the
best course of action I can think of is to remove this functionality.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
