Deny unused crate dependencies (#549)

* subxt unused crate deps

* Deny unused dependencies

cli/src/main.rs

@@ -14,6 +14,8 @@
 // You should have received a copy of the GNU General Public License
 // along with subxt.  If not, see <http://www.gnu.org/licenses/>.
 
+#![deny(unused_crate_dependencies)]
+
 use color_eyre::eyre::{
     self,
     WrapErr,