Remove `enable-serde` feature on compiler types

[xdoardo]

Some of the "compiler types" we use (e.g. Relocation, RelocationKind,…) are marked to implement serde's traits depending on the enable-serde feature being enabled. It seems that this feature is not needed anymore. An example of these types is the following:

#[cfg_attr(feature = "artifact-size", derive(loupe::MemoryUsage))]
// #[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))] <- remove me
#[derive(RkyvSerialize, RkyvDeserialize, Archive, Copy, Clone, Debug, PartialEq, Eq)]
#[rkyv(derive(Debug), compare(PartialEq))]
#[repr(u8)]
pub enum RelocationKind {
  ...
}