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: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240403080702.3509288-6-arnd@kernel.org


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