test: replace the Go compatibility fixtures with native round-trip tests

The Go-generated link.db, profiles.link and rec1.cast fixtures and the
tests that read them are removed. Their coverage is kept by tests on
data this build writes: a fresh database round-trips SSH profiles (env
vars, login scripts, forwards), serial profiles, IRC room state, known
hosts, snippets and preferences, and encrypted exports round-trip and
reject a wrong password. The import dialog no longer offers .link files
and the docs stop mentioning them.

rust/core/src/service.rs

@@ -274,7 +274,7 @@ impl Service {
         Ok(path)
     }
 
-    /// Imports a `.nexus` (or legacy `.link`) export.
+    /// Imports a `.nexus` export.
     pub fn import_encrypted(&self, path: &Path, password: &str, strategy: &str) -> Result<usize> {
         let ps = exportimport::decrypt_and_import(path, password)?;
         self.import(ps, strategy)