Remove argument sq key subkey export --cert-file.

- This doesn't work, and it is of questionable use.  Also, `sq key
    export` doesn't have `--cert-file` either.

  - Fixes #464.

NEWS

@@ -120,6 +120,7 @@
    - The argument `sq cert lint --export-secret-keys` has been
      removed: if a secret key is provided as file input, it will be
      emitted.
+   - The argument `sq key subkey export --cert-file` has been removed.
 
 * Changes in 0.39.0
 ** Notable changes

src/cli/key/subkey/export.rs

@@ -45,7 +45,7 @@ certificate, use `sq key export`.
 pub struct Command {
     #[command(flatten)]
     pub cert: CertDesignators<
-        cert_designator::CertUserIDEmailFileArgs,
+        cert_designator::CertUserIDEmailArgs,
         cert_designator::CertPrefix,
         cert_designator::OneValue>,
 

src/cli/types/cert_designator.rs

@@ -160,6 +160,12 @@ pub type CertUserIDEmailFileArgs
         as std::ops::BitOr<EmailArg>>::Output
        as std::ops::BitOr<FileArg>>::Output;
 
+/// Enables --cert, --userid, and --email (i.e., not --domain,
+/// --grep, --file, --with-password, or --with-password-file).
+pub type CertUserIDEmailArgs
+    = <<CertArg as std::ops::BitOr<UserIDArg>>::Output
+       as std::ops::BitOr<EmailArg>>::Output;
+
 /// Enables --cert, --userid, --email, --file, --with-password and
 /// --with-password-file (i.e., not --domain, or --grep).
 pub type CertUserIDEmailFileWithPasswordArgs