efivarfs: Remove unused internal struct members

The structure was moved to the efivarfs internals in commit 2d82e622
("efi: vars: Move efivar caching layer into efivarfs") after previously
being used as the data ABI for efivars until its removal in commit
0f5b2c69 ("efi: vars: Remove deprecated 'efivars' sysfs interface").

As efivarfs only uses the structure for the variable name caching layer,
the data-related members were never in use. Remove them to avoid
implying that efivarfs is bound by the same restrictions that efivars
once had. While at it, remove the packed attribute, since we no longer
have to guarantee a stable layout.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>