greybus: eliminate unneeded null check

Coccinelle points out that debugfs_remove_recursive() handles a null
argument properly, so there's no need to check for NULL before
making the call.  I have verified this is true of the kernel we're
now working with (arche-6.0).

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>