fix(tvix/nix-compat-derive): Get rid of external feature flag

The external feature flag was there because I couldn't find a way to
refer to crate and nix-compat with the same name so that the generated
code could be the same.

In essence `use nix_compat::nix_daemon:🇩🇪:NixDeserialize` is an error
when used inside nix_compat crate.

So my best fix was the external feature flag until I found the solution
used here which also removes the flag completely.

Change-Id: Ia3e89c6c350c3fb22ca87f974a39c21542aae152
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12376
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>
Autosubmit: Brian Olsen <me@griff.name>

tvix/nix-compat-derive/Cargo.toml

@@ -6,11 +6,6 @@ edition = "2021"
 [lib]
 proc-macro = true
 
-[features]
-external = []
-default = ["external"]
-
-
 [dependencies]
 proc-macro2 = { version = "1.0.86",  features = ["proc-macro"] }
 quote = { version = "1.0.36", features = ["proc-macro"] }