hw/net/cadence_gem: Remove dead code

hw/net/cadence_gem: Remove dead code

Commit 191946c moved the code to handle padding to minimum
length from after the handling of the CRC to before it.
This means that the CRC code doesn't need to cope with the
possibility that the size is less than 60; remove this
dead code.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>