efivarfs: Drop redundant cleanup on fill_super() failure

Al points out that kill_sb() will be called if efivarfs_fill_super()
fails and so there is no point in cleaning up the efivar entry list.

Reported-by: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>