net: Kconfig: remove obsolete reference to config MICROBLAZE_64K_PAGES

Commit 05cdf457

 ("microblaze: Remove noMMU code") removes config
MICROBLAZE_64K_PAGES in arch/microblaze/Kconfig. However, there is still
a reference to MICROBLAZE_64K_PAGES in the config VMXNET3 in
./drivers/net/Kconfig.

Remove this obsolete reference to config MICROBLAZE_64K_PAGES.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>