Test: remove cargo clean

Author: Alexander-N

ci/travis/test.sh

@@ -1,7 +1,6 @@
 #!/bin/bash
 set -ex
 
-cargo clean
 cargo test --features "$FEATURES num-complex test-doc"
 if [ $TRAVIS_JOB_NAME = 'Minimum nightly' ]; then
     cargo fmt --all -- --check