bsd-user: Remove reference to CONFIG_UNAME_RELEASE

bsd-user: Remove reference to CONFIG_UNAME_RELEASE

Commit e586822a5 broke the bsd-user build when it removed the
CONFIG_UNAME_RELEASE define but forgot to remove the use of it
in bsd-user. Fix this in the simplest possible way (bsd-user
doesn't make any use at all of the qemu_uname_release variable
except to allow it to be pointlessly set by the user, so this
is all we need to do.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1399648001-20980-1-git-send-email-peter.maydell@linaro.org