gve: Remove unused gve_adminq_set_mtu

The last use of gve_adminq_set_mtu() was removed by
commit 37149e93 ("gve: Implement packet continuation for RX.")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Kalesh AP <kalesh-anakkur.purayil@broadcom.com>
Reviewed-by: Praveen Kaligineedi <pkaligineedi@google.com>
Link: https://patch.msgid.link/20241211001927.253161-1-linux@treblig.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>