3c515: remove unused 'mtu' variable

This has never been used since the start of the git history. When building
with W=1, the unused variable produces a gcc warning:

drivers/net/ethernet/3com/3c515.c:35:18: error: 'mtu' defined but not used [-Werror=unused-const-variable=]

Just remove it.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/ethernet/3com/3c515.c

···
    Setting to > 1512 effectively disables this feature. */
 static int rx_copybreak = 200;
 
-/* Allow setting MTU to a larger size, bypassing the normal ethernet setup. */
-static const int mtu = 1500;
-
 /* Maximum events (Rx packets, etc.) to handle at each interrupt. */
 static int max_interrupt_work = 20;