ci: remove cargo clean

We aren't using Travis' Cargo cache any more (because it actually seems
to slow down builds), so there's no reason to clean out old build
outputs.

Also, even if we were using the Cargo cache, our approach to finding the
correct Cargo OUT_DIR has become more robust, so we still wouldn't need
to remove the old build outputs.

ci/script.sh

@@ -7,9 +7,6 @@ set -ex
 . "$(dirname $0)/utils.sh"
 
 main() {
-    # Travis sometimes caches the target directory, which makes testing the
-    # output of cargo a little trickier. So just wipe it.
-    cargo clean
     # Test a normal debug build.
     cargo build --target "$TARGET" --verbose --all