Replace serde_derive with serde/derive

[kpcyrd]

Debian autopkgtests is very eager in trying to find all possible combinations, and tries to build the crate with cargo check --no-default-features --features serde, which currently doesn't work.

I'm patching this downstream, this change removes the need for that patch.

In theory this is a breaking change, in case somebody uses (for whatever reason):

irc = { version = "1", features = ["toml_config", "serde_derive"] }

this would not work anymore, you could work around this by having a feature named "serde_derive" that does nothing:

[features]
serde_derive = []

[aatxe]

I think I'm okay with this, derive was never really intended on being optional separately from serde, but uhh... Hyrum's law, I suppose. So, hello to whoever this PR broke.