greybus: operation: drop redundant oom message

Drop redundant OOM message, which would already have been logged by the
memory subsystem.

Also remove a FIXME about possibly sending a pre-allocated response,
which is neither possible or desirable. If we ever run out of memory we
have bigger problems then getting a response back to firmware.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>