efivarfs: remove unused efivarfs_list

Remove all function helpers and mentions of the efivarfs_list now that
all consumers of the list have been removed and entry management goes
exclusively through the inode.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>