netdevsim: remove max_vfs dentry

Commit d3953819 ("netdevsim: Add max_vfs to bus_dev")
added this file and saved the dentry for no apparent reason.

Link: https://lore.kernel.org/r/20211028211753.22612-1-kuba@kernel.org


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