`skip_deserializing` required even when not deriving `Decode`

[realbigsean]

If you try to derive just Encode on a struct, using transparent, you are required to add skip_deserializing to all but one struct field, even if you aren't deriving Decode.

Example:

#[derive(Debug, Clone, PartialEq, Encode)]
#[ssz(struct_behaviour = "transparent")]
pub struct RuntimeVariableList<T: Encode> {
    vec: Vec<T>,
    #[ssz(skip_serializing)]
    max_len: usize,
}

The above results in:

error: proc-macro derive panicked
   --> beacon_node/lighthouse_network/src/rpc/methods.rs:346:35
    |
346 | #[derive(Debug, Clone, PartialEq, Encode)]
    |                                   ^^^^^^
    |
    = help: message: A "transparent" struct must have exactly one non-skipped field (2 fields found)

error: could not compile `lighthouse_network` (lib) due to previous error

Whereas the following compiles fine:

#[derive(Debug, Clone, PartialEq, Encode)]
#[ssz(struct_behaviour = "transparent")]
pub struct RuntimeVariableList<T: Encode> {
    vec: Vec<T>,
    #[ssz(skip_serializing, skip_deserializing)]
    max_len: usize,
}